Mysterious Whine... mechanic is stumped. Any Ideas?

Hello everyone. I've been a lurker on the forums for a while, since I got my Jeep in May. I must warn that this is going to be a hell of a long post. First off, the specs:

2001 Cherokee Sport w/ 138k miles
4.0L
AW4 transmission
NP231 TC
8.25" rear end
edit: I forgot to mention that the Jeep is bone stock save for larger tires (235/75 vs. the 215/75 that came stock, if memory serves)

Here's what's going on - Shortly after I bought my Jeep I noticed a slight whine from what sounded like the rear end at 45 mph during cruise. It was so quiet that I could only hear it with the radio turned off and the windows up. It would only be present at cruise and would change pitch with speed, though it was a small window where I could hear it. I changed the oil in the rear end (along with every other fluid in the truck) as part of making it mine. A few weeks later the whine started at about 35 mph and continued to about 50. A couple weeks later it had continued to about 60.

Eventually I had a friend of mine said there was something wrong with the rear axle, so I swapped in a used axle with 65k on it. Changed the gear oil out and put synthetic 75W140 (I was planning on doing some towing) and took it for a ride. Now the noise (which I thought was coming from the new axle, but I’ll cover that in a minute) has changed a bit – the whining is still there from 35 – 60 and has slowly gotten louder. In addition, there was now a howling/whirring noise during deceleration at any speed above 35 or so, changing pitch with speed. I couldn’t return the used axle so I lived with it for a while (about 2500 miles) and it never seemed to get any worse but it bugged the hell out of me.
I brought it to a trusted mechanic who thought it might be a pinion bearing because of how the sound came and went with coast but wasn’t sure. He opened up the axle, found some rust in the front pinion bearing and figured he’d found the problem. I had him replace all of the bearings (both pinion and both carrier bearings). He reset the gears and refilled the case, again with 75W140. I took it out for a spin and neither sound changed at all. They were both still there!

I brought the jeep back to the mechanic to try to find the noise again. He rechecked the rear end and said everything still looks good and that it’s in spec (he mentioned the gears looked fine, bearings are new and look fine, etc). He took it for several rides and was able to replicate the noise but it seems to change positions (from directly under the driver at low speeds to the rear end at higher speeds) and he couldn’t tell where it was coming from. He had 3 other mechanics look at it as well with no conclusion. Also, it apparently doesn’t make any strange noises while up on the rack no matter what speed.

The mechanic asked me to drive it for a while longer to see if the noise changes or if it gets louder. When/if it does, then bring it back to him and he’ll go for a ride with me to try to figure it out. The only suggestion he had is that it might be the transfer case, but he said that wasn’t much more than a shot in the dark (Side note: the transfer case seems somewhat noisy in 4HI… it kind of has a whir to it but doesn’t seem major. BUT, I have nothing to compare it to because I’ve only otherwise driven cars. Also, the fluid was dark red and seemingly clean when I changed it)

Cliff notes:

• Whining noise at 35-60 during cruise/acceleration
• Whirring/howling noise while decelerating from speeds above 35 mph
• The whining noise has existed with 2 different rear axles
• The whirring/howling noise has only happened with the replacement axle BUT persisted AFTER a replacement of all of the bearings.

I’ve done a ton of searching but no conclusions. Could it be the transfer case? Transmission (which otherwise seems fine)? Please help! Thanks a lot!

Brakes?

The front brakes had been removed and reinstalled by a mechanic when I had the 2 front axle u-joints replaced. I replaced most of the hardware on the rear brakes when I swapped in the new axle (the shoes had plenty of life left). The mechanic also had to remove/reinstall rear brakes when rebuilding my rear axle.

Plus, the noise isn't effected by braking, only throttle position. Thanks for the suggestion though!

hmm, I hate issues like this! My thoughts (sorry if they are redundant):

-Driveshaft U-joints can make preety weird noises even if they do not have play, just due to bad bearings in the caps.

-Of course the trans or transfer case could be the issue but I'm not sure how you would isolate that exactly.

I would say that if you replaced the rear end and it has still continued, it's another issue than the axle. If you somehow got a second axle that has the same noises, go buy a lottery ticket! Pull your rear driveshaft, and make sure the u-joint caps spin nice, and have no play with the drive shaft installed. If you are good at detecting the location of noises, and it from the back, this is where I would turn first.

Good luck and keep us posted!

Have you checked the fluid in the tcase? Since you swapped axles, I wouild be real surprised if the exact same problem was in the replacement axle.

My thoughts run to the tcase. I would also look for a bad tranny mount. Now having said all that, I have had tires that had more of a whine than a roar as they aged.

Suspension noises can be hard to pinpoint. I had a noise I would have sworn was in the rear. It turned out to be a bad unit bearing on the front axle.

I might think about dropping the driveshaft and running in 4wd to see if the noise changes.

drepamig said:

Also, it apparently doesn’t make any strange noises while up on the rack no matter what speed.

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Please clarify this statement. Do you mean he had it on a dyno of some sort?

Thanks for all of the suggestions! To answer some of the questions:

crimsondragon: I meant when it was up on the lift in the garage with the wheels freely spinning in the air. It seems as though the noise only happens when there's a load on the drivetrain

old_man: I changed the oil in the TC in maybe June or July. The old fluid was very dark red, but smelled "normal". I didn't notice any shavings in it. I put mobil 1 synthetic (atf I think, if memory serves... whatever it was I made sure it was recommended on various Jeep forums) back in there. The mechanic checked the fluid and it was a touch low but nothing dangerous he said. I have a weeping seal on the TC it seems.

noChrome71: I literally have a u-joint for the drive shaft sitting on my back seat right now but just haven't had time to replace it yet. A combination of working 60 hrs/week and that it's been in the low 30s with no garage. That's my next task though.

As for driving without drive shafts - I forgot to mention that I removed the rear drive shaft in August (before the rear end was rebuilt) and the whir I usually get while decelerating was continuous but much quieter and seemed to be coming from the rear of the Jeep. It made sense to me at the time because I was convinced it was a pinion bearing. I figured that the front pinion was loaded since the axle was basically being back-driven by the wheels, just like in decel.

I've never taken the front drive shaft out to see what happens. It seems more difficult than the rear and I live in far northern Michigan (the upper peninsula actually), so firmly in rust country. I don't want to strip anything out. Though, to be honest, I've never even tried taking it out.

Looks like I need to replace that u-joint and see what happens. Thanks for all of the suggestions! If there are any other ideas, I'm all ears.

Also - what is a unit bearing? edit: never mind, google told me the answer

Another possibility that I'm not sure if it exists or could cause the problem but do either or both of the axles you've had in the jeep have a LSD? If so did you add the friction modifier to the diff fluid?

Tires-rotate them and/or lower/raise the air pressure to see if the noise changes any.

dave92cherokee said:

Another possibility that I'm not sure if it exists or could cause the problem but do either or both of the axles you've had in the jeep have a LSD? If so did you add the friction modifier to the diff fluid?

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Both the new and old axle have/had open differentials, so I didn't put any friction modifier in the oil.

xjtrailrider said:

Tires-rotate them and/or lower/raise the air pressure to see if the noise changes any.

Click to expand...

The initial whining noise (the one in cruise at or around 45mph) existed with the set of tires that came with the car. I put new ones on and it made no difference. Also, rotating, balancing and/or changing pressure on the new tires hasn't made any difference.

Thanks for the suggestions!

drepamig said:

The initial whining noise (the one in cruise at or around 45mph) existed with the set of tires that came with the car. I put new ones on and it made no difference. Also, rotating, balancing and/or changing pressure on the new tires hasn't made any difference.

Thanks for the suggestions!

Click to expand...

Not a tire issue then. Try this; Pull the rear drive shaft, pack the inside of the slip yoke with some Lucas Red-n-Tacky grease and re-install it. See if the noise goes away for a little while anyway. If it does then i would replace the slip yoke with a new one. When the slip yoke or the output shaft starts getting wear in them, they will make all kinds of noises.

One of the first things I learned about XJ's when I switched from CJ's is that our beloved unibody Jeeps will resonate noises from the strangest things and amplify it. That is one of the drawbacks of not having the body isolated from the frame.

xjtrailrider said:

Not a tire issue then. Try this; Pull the rear drive shaft, pack the inside of the slip yoke with some Lucas Red-n-Tacky grease and re-install it. See if the noise goes away for a little while anyway. If it does then i would replace the slip yoke with a new one. When the slip yoke or the output shaft starts getting wear in them, they will make all kinds of noises.

One of the first things I learned about XJ's when I switched from CJ's is that our beloved unibody Jeeps will resonate noises from the strangest things and amplify it. That is one of the drawbacks of not having the body isolated from the frame.

Click to expand...

Ok, thanks for the suggestion! I'll have to give that a shot when I have my drive shaft out. I have previously pulled the slip yoke out and regreased it which got rid of the typical clunk late model XJs get when stopping, though that didn't make a difference for the whine. But I used Kendall Blue grease, not the stuff you suggested. However, I do still have a clunk when shifting from park to drive that doesn't seem to come from any play in my u-joints.

My 99 has a similar type whine under acceleration, I also thought it was the rear end until I replaced everything with F/R lockers and new bearing install. Then the AA SYE kit and woods rear driveshaft.

I know what you mean because it really sounds like it's coming from the rear end under acceleration load but I'm about 90% sure it's coming from the transfer case. In my case I was in a hurry and didn't bother to replace all the bearings when I did the SYE and that's why I think it's coming from there (150k on mine).

I think my transfer case mount but there's a comment up above about it may be the transmission mount, I'll have to try that since mine is probably the original and I know it's a little squashed.
It hasn't really gotten any worse so until it does I'm not breaking the case back apart.

I had a similar noise and found out that all the nuts holding the t case to the tranny were loose, once i tightened them up I started getting some vibes at about 60 mph. I notice my u joint has a little play under the straps at the rear end (looks like one cap has been spinning), so u joints and straps will be replaced next.

My real question is how much play is acceptable in the output shafts of the t case? I noticed both sides have a little play when I push/pull on them.

Does this sound familiar?
http://www.naxja.org/forum/showthread.php?t=1044693&highlight=whirring

Well I replaced the rear DS U Joints today and also found that the nut on the TC output was a little loose. The vibes have dropped about in half. Tomorow the front DS is coming out to check everything on that side.

If the output shaft babbit bearing is shot in the tail cone, I probably have a replacement in my stockpile.

I'm really not sure, tightening the nut seemed to take the play out of it. I guess I'll have to keep an eye on it.

My bet... check your exhaust hangars, as well as engine/tranny mounts and check clearance between the exhaust and the floor, tranny crossmember, muffler/tailpipe to rear diff housing, and tailpipe to gas tank, trailer hitch, and leaf shackle clearance. Look for shiny spots that indicate rubbing.

Try holding the brake and gassing it a little at a stop light and see if the sound changes or goes away.