Diagnosis Help -- Hum After 40+ mph

[Kayphix]

Recently noticed a hum on my 1999 LC with 247k miles after 40mph. Letting off the accelerator removes the hum for a split second only to return while the truck is losing speed. Switching to neutral does not remove the noise.

I just changed the passenger side wheel bearings and will do the driver this weekend. I was told this is the first place to check (thinking it was coming from that side originally).

Any other ideas? Was debating rotating tires and changing differential/transmission fluids next.

Recently got: new CV axles, brake master cylinder/accumulator, new radiator/fan bracket and shroud and rear calipers

 

[awesomeissquid]

Has it always hummed? Assuming the hum was occurring before all the other work was done? What tires do you have and is it possible this is just tire noise?

If you have the car stationary and you shift into neutral and rev the engine I assume the noise does not occur at any of the standard RPMs you typically see it at.

If all that isn’t it, then my guess would be more for differential or transmission related. Reason being that the wheel bearings are spinning regardless of engine input or not. So I would not expect them to have a momentary reduction in noise when you let off the throttle, only for it to return a few seconds later as the driveline decelerates. The transmission and differentials or maybe even the CVs would have some sort of momentary slack until the deceleration catches up and puts some force on them. I think you are on the right track with swapping fluids first since you will be able to see how the fluids look and if the there are any particles or shavings present. After ensuring they are properly filled and driving again, if the noise is still present, maybe consider getting one of the mechanics microphones that has all the different test leads so you can try to pin down the source more closely.

 

[trainbufftony]

Dump fluids and inspect in the sunlight for glitter. Also take the driveline out and check the u joints by hand, and the slip joint too. They can get a bind and make noise, but feel tight in the truck.
Pull em out check them by hand.

 

[Kayphix]

Has it always hummed? Assuming the hum was occurring before all the other work was done? What tires do you have and is it possible this is just tire noise?

If you have the car stationary and you shift into neutral and rev the engine I assume the noise does not occur at any of the standard RPMs you typically see it at.

If all that isn’t it, then my guess would be more for differential or transmission related. Reason being that the wheel bearings are spinning regardless of engine input or not. So I would not expect them to have a momentary reduction in noise when you let off the throttle, only for it to return a few seconds later as the driveline decelerates. The transmission and differentials or maybe even the CVs would have some sort of momentary slack until the deceleration catches up and puts some force on them. I think you are on the right track with swapping fluids first since you will be able to see how the fluids look and if the there are any particles or shavings present. After ensuring they are properly filled and driving again, if the noise is still present, maybe consider getting one of the mechanics microphones that has all the different test leads so you can try to pin down the source more closely.

The hum started occuring about 3 weeks ago, a few weeks after it got all that work complete. That's when I started my research and saw to replace wheel bearings first.

I'll report back after I swap the fluids. Appreciate the response.

 

[Kayphix]

Basically this thread can be deleted.

Took it in to a shop to diagnose the sound.
Guess I need my differentials rebuilt at some point. Mechanic said it's not urgent but it should be added on my to do list. Makes sense, 247k miles on her.

 

[awesomeissquid]

How did they come to the conclusion that the differentials need to be rebuilt? Did they say both need it? That seems odd.

 

[Kayphix]

How did they come to the conclusion that the differentials need to be rebuilt? Did they say both need it? That seems odd.

I'll figure that out once I get it. I assumed they gave me the quote for both just because the age of the vehicle.

I plan on getting a second opinion next week. However, the current place hasn't done me wrong yet.

 

[trainbufftony]

Usually a failing 3rd member has a different noise on the throttle vs off the throttle. The carrier is loading a different bearing which sounds different.
wheel bearings will make constant noise and pinion bearings can usually be checked by hand...

 

[Mike NXP]

Basically this thread can be deleted.

Took it in to a shop to diagnose the sound.
Guess I need my differentials rebuilt at some point. Mechanic said it's not urgent but it should be added on my to do list. Makes sense, 247k miles on her.

=] lol

I like how you opened with "debating rotating my tires" and that led to a possible diff rebuild. Reading these forums and talking to crappy mechanics tend to get people to fear the worst case scenario.

Got some road noise? Start with the basic s***, your tires are probably unevenly worn and might be as simple as that. If that's not the case, maybe you ask yourself if this is even a problem or just a noise you are now focusing on. From there you can start diving deeper and thinking about fluids, wheel bearings, and what not.

An easy way to diagnose a bad wheel bearing is to turn heavily to one side while in motion and see if the hum gets louder of quieter. That would be a sign of a bad wheel bearing. If the noise doesn't change then it's probably not a bearing problem. I just replaced my bearings at 225k miles and they looked new still. They don't fail often unless they run dry and overheat.

As for gear oil / fluids, you can just crack the fill and dip your finger in to get an idea of condition of fluid.


I guess I'm just saying start simple and then work your way in deeper. Road noise and humming over certain speeds is almost always tire noise. If it was a gear / transmission type of hum you would know.


Ask that mechanic if you agree to rebuild the diff as they suggested and the noise doesn't go away, if he will pay the bill. I bet his diagnosis will change.

 

[Kayphix]

=] lol

I like how you opened with "debating rotating my tires" and that led to a possible diff rebuild. Reading these forums and talking to crappy mechanics tend to get people to fear the worst case scenario.

Got some road noise? Start with the basic s***, your tires are probably unevenly worn and might be as simple as that. If that's not the case, maybe you ask yourself if this is even a problem or just a noise you are now focusing on. From there you can start diving deeper and thinking about fluids, wheel bearings, and what not.

An easy way to diagnose a bad wheel bearing is to turn heavily to one side while in motion and see if the hum gets louder of quieter. That would be a sign of a bad wheel bearing. If the noise doesn't change then it's probably not a bearing problem. I just replaced my bearings at 225k miles and they looked new still. They don't fail often unless they run dry and overheat.

As for gear oil / fluids, you can just crack the fill and dip your finger in to get an idea of condition of fluid.


I guess I'm just saying start simple and then work your way in deeper. Road noise and humming over certain speeds is almost always tire noise. If it was a gear / transmission type of hum you would know.


Ask that mechanic if you agree to rebuild the diff as they suggested and the noise doesn't go away, if he will pay the bill. I bet his diagnosis will change.

I started with wheel bearings. Just replaced and repacked as it seemed like a good novice mechanic work to do. Didn't fix the hum.

I then rotated tires. Nothing. Changed diff/transfer case fluids. Nothing.

The thing that originally led me to believe some sort of driveline noise was when I lifted my foot off the accelerator, the noise disappeared for a split second. Tires should be a constant hum.

I plan on getting a second opinion next week.

 

[Mike NXP]

I started with wheel bearings. Just replaced and repacked as it seemed like a good novice mechanic work to do. Didn't fix the hum.

I then rotated tires. Nothing. Changed diff/transfer case fluids. Nothing.

The thing that originally led me to believe some sort of driveline noise was when I lifted my foot off the accelerator, the noise disappeared for a split second. Tires should be a constant hum.

I plan on getting a second opinion next week.

Copy that..... maybe take a few tred depth measurements, just hoping it's something as simple as weird tire wear.


Do you feel any vibrations or just hear a noise?

 

[Kayphix]

Copy that..... maybe take a few tred depth measurements, just hoping it's something as simple as weird tire wear.


Do you feel any vibrations or just hear a noise?

I haven't felt any weird vibrations other than what I assume to be 24 year old rig especially when it reaches 70mph in the steering wheel itself - I've had it for 6k miles myself, so I would say it's nothing new in comparison to the hum noise.

I haven't been able to get my LC back yet. Just got the 12" of snow in Denver but I'll be sure to check the tire tread depth.

The mechanic (reputable 4x4 shop) originally told me it was the tires making the noise after rotation. I told them I rotated the tires to try fixing the noise. That's when they called back and said you aren't crazy, there is some driveline noise.

 

[Mike NXP]

I haven't felt any weird vibrations other than what I assume to be 24 year old rig especially when it reaches 70mph in the steering wheel itself - I've had it for 6k miles myself, so I would say it's nothing new in comparison to the hum noise.

I haven't been able to get my LC back yet. Just got the 12" of snow in Denver but I'll be sure to check the tire tread depth.

The mechanic (reputable 4x4 shop) originally told me it was the tires making the noise after rotation. I told them I rotated the tires to try fixing the noise. That's when they called back and said you aren't crazy, there is some driveline noise.

Interesting, shoot so maybe you are up against something worse off but still have my fingers crossed it's simple. Keep us posted.

 

[2001LC]

Basically this thread can be deleted.

Took it in to a shop to diagnose the sound.
Guess I need my differentials rebuilt at some point. Mechanic said it's not urgent but it should be added on my to do list. Makes sense, 247k miles on her.

Which shop?

Whereas the 98-99 do have a weak frt differential. They blow and need replacing, from aggressive off-road (bouncing off rocks). Hum is not usually a concern.

Have the aft & forward propeller shafts slip yokes and spiders (AKA drive shaft & u-joints) been lubed ever 5K miles. Pictures of them along with area around and above them, can be revealing. Under-lubed spiders will hum, but typically and oscillating sound. A good test is east bound I-70 dropping into Denver. At ~60 MPH shift to N and coast. If you hear WOW WOW WOW, Lube them see what happens!

Replace gear lube is important. Just as import is which lube and setting level correctly. I use Mobil 1 75-90W.

Before I'd recommend R&R or rebuild of a gear box. I'd make sure all fluid/lubes as they should be.

 

[Kayphix]

Which shop?

Whereas the 98-99 do have a weak frt differential. They blow and need replacing, from aggressive off-road (bouncing off rocks). Hum is not usually a concern.

Have the aft & forward propeller shafts slip yokes and spiders (AKA drive shaft & u-joints) been lubed ever 5K miles. Pictures of them along with area around and above them, can be revealing. Under-lubed spiders will hum, but typically and oscillating sound. A good test is east bound I-70 dropping into Denver. At ~60 MPH shift to N and coast. If you hear WOW WOW WOW, Lube them see what happens!

Replace gear lube is important. Just as import is which lube and setting level correctly. I use Mobil 1 75-90W.

Before I'd recommend R&R or rebuild of a gear box. I'd make sure all fluid/lubes as they should be.

Parker 4x4. They've done a lot of my work, cause their 24k mile/2yr warranty. I did contact Japanese Auto in Wheat Ridge, waiting for them to respond for a second opinion.

I've been lubing the drive shaft since I bought it (7k miles ago). So far, that's been twice.

Mine is more a constant hum in between 40-60, after 60 wind/tire noise take over so you either A) can't hear it or B) it goes away?

I changed all diff fluids, transfer case and transmission fluid was done 5k miles ago. They weren't even that bad in color. Tire rotation was complete before taking it to the shop. I don't hear anything under the truck while it's idling. Driveshaft doesn't move when the truck is off.

 

[Mike NXP]

Sorry to focus so much on tires but what type are you running? how does the tread wear look? Generally speaking a hum without any floor / wheel vibration is usually road noise, if something mechanical was failing you would probably feel it and it.

You said this rig is new to you, about 6kmiles so far? Was the hum always present? Could it be normal ?

 

[Kayphix]

Sorry to focus so much on tires but what type are you running? how does the tread wear look? Generally speaking a hum without any floor / wheel vibration is usually road noise, if something mechanical was failing you would probably feel it and it.

You said this rig is new to you, about 6kmiles so far? Was the hum always present? Could it be normal ?

Hum was not always present. Literally started after Thanksgiving Holiday. The only reason I know it's a new sound is because of the lifting of the accelerator into deceleration. The sound for a split second goes away as soon as you take your foot off. I took the truck on a 3k mi roadtrip in October and didn't notice that ever.

Running Goodyear Wrangler Duratracs. When I bought it, I would say they had 80% tread on it, so probably now 60%? I still need to get it back but currently snowed in.

 

[Mike NXP]

Hum was not always present. Literally started after Thanksgiving Holiday. The only reason I know it's a new sound is because of the lifting of the accelerator into deceleration. The sound for a split second goes away as soon as you take your foot off. I took the truck on a 3k mi roadtrip in October and didn't notice that ever.

Running Goodyear Wrangler Duratracs. When I bought it, I would say they had 80% tread on it, so probably now 60%? I still need to get it back but currently snowed in.

Roger that, I wouldn't rule out tire noise especially on something meaty like a duratrac with some mileage on em' Even though the noise wasn't there at first, 3k miles on highway with perhaps not proper pressure could cause some slight wear and a hum.

I wonder if locking the center diff would have any change in symptoms and point you towards something driveline related.

If you wanted to focus on driveline related stuff you could ask mechanic to disconnect front driveline. There are some forums on here about making our rigs temporary 2 wheel drive. This would eliminate the front end from the possible suspects.

 

[Kayphix]

Roger that, I wouldn't rule out tire noise especially on something meaty like a duratrac with some mileage on em' Even though the noise wasn't there at first, 3k miles on highway with perhaps not proper pressure could cause some slight wear and a hum.

I wonder if locking the center diff would have any change in symptoms and point you towards something driveline related.

If you wanted to focus on driveline related stuff you could ask mechanic to disconnect front driveline. There are some forums on here about making our rigs temporary 2 wheel drive. This would eliminate the front end from the possible suspects.

Got the Cruiser back. Unfortunately my mechanic was sick with the fly so I couldn't ask the issue.

However I checked tire tread depth. 6/16" - 1/2" still left for these Wranglers.

Also, disregard what I said about the noise disappearing for a split second when releasing the accelerator. It actually becomes a higher pitch for a split second then leveling back off to it's standard hum noise.

 

[ibrahim83]

Got the Cruiser back. Unfortunately my mechanic was sick with the fly so I couldn't ask the issue.

However I checked tire tread depth. 6/16" - 1/2" still left for these Wranglers.

Also, disregard what I said about the noise disappearing for a split second when releasing the accelerator. It actually becomes a higher pitch for a split second then leveling back off to it's standard hum noise.

Drop your rear drive shaft and inspect your u joints for wear.