Help and Expertise needed: Mystery Noise in 80 Series (1 Viewer)

[Nathan King]

Hey. I need some help with my 1996 80 Series with 412,000 miles on it. I've got a loud rattle/grind noise that happens when I let my foot off the accelerator and get into float. If I completely let my foot off the accelerator, the noise does not happen. If I just back off the accelerator a bit, that's when you can hear it. While I am accelerating, no noise.
At highway speeds, the noise is especially pronounced.

Here's what I have done so far:

Thought it was in the transfer case but my mechanic buddy did not, so I replaced the bearings on the rear diff pinion with new seals all around. Pinion depth, backlash, etc all good. Noise did not change at all.

So then we rebuilt the transfer case. The noise really seemed like it was coming right out of there. After that the noise still remains.

Checked all u-joints on the drive shafts. Transmission is shifting fine and the fluid is the right color. I check the diff oils and t-case oil (and top them off when needed) every two weeks.

What am I missing? Has anyone had this issue? What should I try next?

Thanks in advance for your help!

 

[Heckraiser]

Worn slip yoke, most likely in the rear driveshaft. Mine surfaced around 310k miles and I recently fixed it at 335k miles with a new driveshaft. Slip yoke is not servicable (other than filling it with grease). Once it gets to the point it starts making the slack-throttle noise, the driveshaft needs to be replaced, or you can just enjoy the noise for several years like I did, as I tried and failed to diagnose the problem I chased u-joints, front shaft, T-case, differential, just like you. Was surprised to find such a simple fix.

:edit: and congrats on the epic mileage!

 

[Nathan King]

Worn slip yoke, most likely in the rear driveshaft. Mine surfaced around 310k miles and I recently fixed it at 335k miles with a new driveshaft. Slip yoke is not servicable (other than filling it with grease). Once it gets to the point it starts making the slack-throttle noise, the driveshaft needs to be replaced, or you can just enjoy the noise for several years like I did, as I tried and failed to diagnose the problem I chased u-joints, front shaft, T-case, differential, just like you. Was surprised to find such a simple fix.

:edit: and congrats on the epic mileage!

Are you serious?!?

 

[Heckraiser]

Very serious Not saying that's absolutely what your problem is, but I chased that noise for years and that's what did the trick.

 

[Nathan King]

Very serious Not saying that's absolutely what your problem is, but I chased that noise for years and that's what did the trick.

Was the wear visible to the eye?

 

[Heckraiser]

I couldn't see it and couldn't really feel it either by wiggling the shaft. But I read about it somewhere here on mud and after exhausting all the other routes, gave it a try.

The noise started as a rattle/growl at slack throttle, as you describe. Over time it would also make the noise when coasting downhill, but only intermittently, not consistently. Also seemed to happen while coasting and going over a wave in the pavement, causing the suspension to compress a little. I'll see if I can find some of the threads that pointed me in that direction. It's not a super common problem as it only happens to the cruisers with advanced mileage it seems.

 

[Heckraiser]

I haven't found the other threads yet, but this was my attempt to internet-diagnose:

Funky Noise on Decel and Slack Throttle

 

[Road Apple]

^^This^^ is why I love MUD !!

 

[MDarius]

8 minutes for a reply. It took him years to find the problem, and it took 8 minutes. I would buy that man a star!

 

[jonheld]

Originally diagnosed by Norm Needham on the old 80sCool email list oh so many years ago. Repeated countless times.

 

[Box Rocket]

I concur with @Heckraiser 's conclusion on this. I have chased the noise for years as well, replacing ujoints and pretty much everything other than the driveshaft since the slip yoke didn't "feel" loose. But.....it's likely to be the issue. The other possible culprit is the output bearing in the tcase.

 

[Nathan King]

I couldn't see it and couldn't really feel it either by wiggling the shaft. But I read about it somewhere here on mud and after exhausting all the other routes, gave it a try.

The noise started as a rattle/growl at slack throttle, as you describe. Over time it would also make the noise when coasting downhill, but only intermittently, not consistently. Also seemed to happen while coasting and going over a wave in the pavement, causing the suspension to compress a little. I'll see if I can find some of the threads that pointed me in that direction. It's not a super common problem as it only happens to the cruisers with advanced mileage it seems.

This sounds exactly like mine.

 

[Nathan King]

8 minutes for a reply. It took him years to find the problem, and it took 8 minutes. I would buy that man a star!

I agree. Awesome that cool people are willing to share with others like this.

 

[Nathan King]

I haven't found the other threads yet, but this was my attempt to internet-diagnose:

Funky Noise on Decel and Slack Throttle

I want to follow up on this. It took me a while to get to it, but replacing the rear drive shaft is exactly what was needed. I spent about $700 on other stuff to find out a "gently used" drive shaft I got for cheap was all I needed. Thank you for your help!

 

[Heckraiser]