Noise upon deceleration... differential problem?

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Help needed with a 2001 LX470 with 116k miles that started making noise during deceleration. I've read through forums and have tried to check the possible causes for the noise, and I think I've more or less isolated the problem to the front differential. The sound seems characteristic of a pinion bearing that lost preload (if there is loss of preload it's not so bad that I can feel it by wiggling the pinion flange). A description of the symptoms and what I've done so far is listed below. Would appreciate anyone's thoughts on whether this is enough data to incriminate the front diff. Is there anything else I should check or troubleshoot before spending the time/money on repairing the front diff?

Symptoms
  • Whirring sound (woo-woo-woo) only upon deceleration/coasting
  • Sound remains with engine braking, shifting to neutral, and braking
  • Sound goes away with gas/acceleration
  • Sound frequency and volume matches vehicle speed (most notable at highway speeds)
  • Sound seems to be coming from the front but it’s hard to tell
  • No vibration is felt in the steering wheel

Key Findings (pointing to the front of the vehicle as the source of the noise)
  • The sound is gone after removing the front drive shaft and driving with the center diff locked (i.e., rear wheel drive mode)
  • The sound is present when the rear drive shaft is removed and driving with the center diff locked (i.e., front wheel drive mode)

Road tested and investigated by two professionals
  • Differential specialist: could not guarantee sound was coming from the differentials nor isolate to either the front or rear diff
  • General mechanic: suspected tires as cause of noise

Completed activities (process of elimination to isolate the cause of the noise)
  • Removed the front and rear driveshafts and checked the differential pinion bearing for play by grabbing the pinion flange and pulling it in/out and rocking it side-to-side/up-and-down; no play was felt - no play was felt in the transfer case outputs either.
  • Jacked up the front of the car and rotated the front wheels individually - no unusual noises or drag
  • Balanced and inspected all tires - performed by Costco; noise is still present
  • Swap out front tires, one at a time with spare tire; noise still present
  • “12/6 position” check on all wheel bearings (front wheel bearings were serviced 3k miles ago)
  • Inspected CV axle cir-clips; cir-clip is fully seated and no gapping to the CV axle observed
  • Inspected for loose/rattling parts, like heat shields; nothing found to be loose
  • Update: Inspected and greased front and rear drive shaft u-joints and slip shafts; u-joints felt smooth and did not feel any play (tested by hand only)
The only thing I have left to do is change the front diff oil and inspect the condition of the oil and signs of metal.
 
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Joined
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If it goes away with the front drive shaft removed, I'd point towards those u-joints.
Thanks for pointing that I out! I forgot to include in my list that I had inspected and regreased all of the u-joints when I removed the front and rear drive shafts. They all felt smooth and I didn't notice any play (tested by hand). I'll see if there is a more perceptive test I should do. Maybe it's a case of "when it doubt, change it out" given new u-joints are relatively cheap?
 

GTV

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Thanks for pointing that I out! I forgot to include in my list that I had inspected and regreased all of the u-joints when I removed the front and rear drive shafts. They all felt smooth and I didn't notice any play (tested by hand). I'll see if there is a more perceptive test I should do. Maybe it's a case of "when it doubt, change it out" given new u-joints are relatively cheap?

I believe what @Trunk Monkey is saying is remove the front driveshaft, lock the center diff and drive the vehicle. If the noise goes away take a closer look at the front diff, etc.
 
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If it goes away with the front drive shaft removed, I'd point towards those u-joints.
I second this. If the noise goes away when the driveshaft is removed, that seems like the culprit. Even with the driveshaft removed and the center diff locked, the front diff is still spinning from the front tires/axles. Since it is making noise coasting and decelerating, it should still make noise with or without the driveshaft. Checking the ujoints is one thing, but these drive shafts have slip yokes and if they are not greased a lot, they can cause noise or the infamous front end clunk when shifting into drive.
 
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I second this. If the noise goes away when the driveshaft is removed, that seems like the culprit. Even with the driveshaft removed and the center diff locked, the front diff is still spinning from the front tires/axles. Since it is making noise coasting and decelerating, it should still make noise with or without the driveshaft. Checking the ujoints is one thing, but these drive shafts have slip yokes and if they are not greased a lot, they can cause noise or the infamous front end clunk when shifting into drive.
You guys are giving me hope it's not a diff issue. :) The sound with deceleration did go away with the front driveshaft removed, but it may have been replaced with a different sound. With the front driveshaft removed, I think I was hearing a significantly more faint version of the sound, but now it was constant regardless of deceleration/acceleration (still matching vehicle speed). At this point, I was listening to for every single sound so I don't know if it was part of my imagination.
 
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Try with OD off.
 
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Tried this today. Sound upon deceleration is unchanged whether OD is on or off.

I ordered OEM u-joints for the front driveshaft. Will report back after I install them.
Then that is good: If sound disappeared, then it could be the faulty bearing inside the O/D planeray gear system (which I suffered, ended getting a full trans rebuilt)

Usually front U-joint's don't fail because the front diff is not moving up/down as the rear diff. Just checking is the front diff phased properly?
 
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Then that is good: If sound disappeared, then it could be the faulty bearing inside the O/D planeray gear system (which I suffered, ended getting a full trans rebuilt)

Usually front U-joint's don't fail because the front diff is not moving up/down as the rear diff. Just checking is the front diff phased properly?
Good to know about the planetary gears!

What do you mean by front diff being phased properly?
 
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if youre lifted, check your u-joint angles. had this on an 80 series, with the lift the u-joint angle caused a whine on decel
My LX470 is still on the stock AHC suspension. We take it off road and utilize the “high” AHC setting often, but like @nissanh said, the front driveshaft should be a nearly a constant angle regardless of the AHC height because of the IFS, right?
 
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I botched one of the u-joint yokes when trying to replace the u-joints in the front drive shaft and as luck would have it, the part was on national backorder. I let the car sit and finally drove it again without the front drive shaft this week. The sound was now present regardless of driveline torque. Same sound as before but it was present during coasting, acceleration, and braking. The frequency of the sound follows the vehicle's speed. I estimated the frequency of the sound, and it appeared to match wheel rotation speed (~4 Hz at 25 MPH; 32 in diameter tires).

After waiting for over a month for the part, I ended up getting a used front driveshaft off ebay. It's not in great condition (there is some rotational play in the slip joint splines and one of the u-joints) but I installed it to help with the troubleshooting. I lifted all four wheels off the ground by putting the car on jack stands. Very quickly I heard a noticeable humming sound. This sound was constant unlike the sounds I heard while driving that was pulsing. The table below shows the configurations I tested and which ones had noise. The humming sound occurs ONLY in 4Hi and when ONLY the front or rear wheels are spinning. If both the front and rear wheels are spinning, there is "no" sound (maybe a very slight sound is some cases). I spun only the front wheels by applying the e-brake to hold the rear wheels and spun only the rear wheels by lowering the front and placing the front tires on the ground.

I posted a video below that captures the sound starting and stopping.

Since the noise is present only when either the front or rear wheels are spinning, does this point to the transfer case as the problem? It's hard to isolate the source of the noise when under the car. Seems loudest around the transfer case and to the rear of the vehicle where the spare tire is. This was when the rear wheels were stopped. I could feel the humming vibration in the rear driveshaft.

There are still some things that confuse me:
- For drive testing: Why did the pulsing sound changed from only upon deceleration to all the time?
- For lifted test: Why is the sound a constant hum and not the pulsing sound from the drive test?

Hoping to get ideas on what else I can do to pinpoint the problem! Thanks!

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