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Can't identify groan coming from rear - can anyone help?

AlpMan

NAXJA Forum User
Location
Colorado
Hey guys,

I'm having a strange "groan" coming from what sounds like the rear of the Jeep. It's been around for a few years but getting progressively louder the last few weeks and gotten to a point where I feel needs to be fixed. It's a 95 XJ with the 4.0, auto trans (AW4), NP231 and the Chrysler 8.25. Stock height.

- Starts at about 5 mph and gets louder at about 30-35 and then quieter again at higher speeds
- The sound gets louder as I make a big swooping turns (both left and right)
- Pitch of the sound gets louder as I step on the gas. If I coast it gets quieter (but is still there)
- Sound still happens with either transfer case or transmission in neutral
- The sound is louder on warm days and quiets down when ambient temperatures are cooler
- The sound is consistent with the rotation speed of tires, so sounds like something in the drive-train after the transmission

What I have done:
- Replaced wheel bearings with no change in sound
- Replaced driveshaft U-Joints with no change in sound
- Opened Diff and inspected everything. Looked good, fluid was clean and replaced with heavier 80W-90 quality gear oil and zero change to the sound
- Replace drums, pads and all brake hardware
- Lifted rear of jeep and checked drive-train with stethoscope. Could not identify any rubbing, grinding or weird sounds
- Spun driveshaft by hand and everything is smooth and solid
- Checked transfer case fluid


Any thoughts? Hints on what to troubleshoot further? I am at a loss and don't know how to proceed without spending big money to replace things like the entire rear end, etc. Many thanks for any help!
 
Could the sound be described as howling, look up howling gears, same noise?

It does sound similar, but not quite the same. It's also not consistent as you would expect gears to sound. I just had my wife drive it and jumped in the back, seems like it's really coming from the passenger side rear. I did replace that axle shaft a few weeks back due to the flange being bent, but as with all the other stuff I tried, no change. The sound was the same before and after.
 
Two things I always do when I have the rear end open, one is pry up on the whole carrier assembly with a pry bar. This will tell you how much carrier bearing play you have. The other is I slap the side of the pinion hard with the palm of my hand and try wiggling the pinion up and down with the drive shaft. On some of my XJ's the carrier bearings went out before the wheel bearings did.

May not even be the rear end, sound travels through the drive train and it can fool you. The problem may not be anywhere near the rear end, even though it sounds like the source. One decent mechanic I know changed every wheel bearing, it eventually turned out to be the output bearing in the transmission.
 
8Mud has good advise like usual. :)

Based on your description I would suspect the carrier bearings, but I would rotate tires first to see if the sound moves.
 
Thanks everyone for the great advise. I did already rotate the tires, that didn't do anything. I think I'll put her up on blocks again and listen my way around the whole rear axle with a stethoscope. It's really starting to sound like the carrier bearing. I do a decent amount of work on her myself, but from reading a bit it sounds like a diff rebuild requires some experience. Cheaper to get it done or to source a used axle?

8Mud - great advise, I will make sure to do that every time I open a diff going forward!
 
X2 on swap tires around. I chased what sounded like a bad wheel bearing, only to discover it went away when I put the snow tires on. Bent flange could have caused some cupping of that tire.
 
Thanks everyone for the great advise. I did already rotate the tires, that didn't do anything. I think I'll put her up on blocks again and listen my way around the whole rear axle with a stethoscope. It's really starting to sound like the carrier bearing. I do a decent amount of work on her myself, but from reading a bit it sounds like a diff rebuild requires some experience. Cheaper to get it done or to source a used axle?

8Mud - great advise, I will make sure to do that every time I open a diff going forward!

Carrier bearings aren't cheap to get fixed, you'll save a fortune if its not a built axle. Whatchu got, D35,C8.25,D44, 8.8?

If your patient you could try doing it yourself and really save.
Ide say upgrade to 8.8 and don't look back, They can be had for around 200.
 
I've got the 8.25 and I just called around, people are asking $800+ to replace. Looks like I may go the 8.8 route or rebuild myself. I've got a buddy that told me he's done it before... sounds like it won't be that big of a deal. He's already got all the specialized tools.
 
Mine made a similar noise for 15 yrs, it finally went away with new axles and bearings but since I did them both at the same time???
 
Just the axles and outer wheel bearings. The gears were done almost 20 yrs ago.
 
Just an update. I tore the diff apart this weekend and finally found what was going on. There was the ever so slightest sideways play with the entire carrier. It seems that somehow the carrier bearing cups unscrewed a little (even though the clips where still there) and so when I was going around a corner or giving it gas, the carrier was moving further away from the pinion gear and thus producing the sounds. All the bearings still seemed good, so I just tightened up the carrier bearing cups, adjusted the gears to the proper backlash and put it ll back together. As good as new! Thanks everyone for the help and suggestions.
 
The roller bearings wear and so do the cups. Did you look at the roller bearings? If they are discolored, usually blue or black, sometimes amber they are unlikely to last long. If the surface if the rollers show spalling or flaking they are likely toast in short order. Most all of the roller bearings I've seen are either surface hardened and eventually wear through to softer material and/or the surface hardening gets hot enough to anneal/soften the hardened metal.
 
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