rumble sound at 45 - 55 mph (tires or brakes or differential)

Hello, I am hoping some of you can give me some insight into my troubleshooting (or lack thereof )

1996 Jeep Cherokee, AW4, 4WD, Chrysler 8.25 differential, 3.55, 228K miles, new Jasper engine with 38K, recent rebuilt front end. recent alignment, recent drive shaft balance with new rear u-joint.

For the past year I have been having a low frequency rumble sound (or whump whump sound) that seems to be coming from the rear and appears in the range of 45-55 and goes away after that speed. It peaks at 50 mph and is very reproducible. I also get a slight steering wheel vibration (and dash and rear view mirror). I have had several places look at it and they have suggested as causes:

- noisy tires (Cooper Discoverer H/T's 225/75r15) ---- possible, as I can find lots of complaints about these tires on the web. I had them balanced several times already. No change in noise, very slight change in steering wheel vibration.

- slipping chain in 231J transfer case (Jeep dealer -- weekend crew). I call B.S on this as I drove in in 4wd Lo and I did not hear the noises posted on the web, associated with chain slap. I then removed the front drive shaft and the 45-55 mph noise was still. Since the chain is stationary (I think) in this situation, I feel I can rule out the transfer case. Checked fluid recently for metal and did not see any glimmer. Piece of advice: mechanics on weekend duty are the lowest seniority and least experienced. Go during the week if possible.

- bad differential. The differential is from a very respectable local parts recycler. 91K miles. I was going to use it as a housing for a rebuild (my original one was leaking and no longer even usable). After getting it in his shop and inspecting it, the guy who was going to do the rebuild suggested that I just use it as is since it looked so good on the inside (he measured backlash, etc) and everything was fine. It was quiet at first but that was with different tires, and it may just be coincidence. I have put less than 10K on it since he inspected it.

The problem I have is that places want to run the vehicle hanging in the air, which can lead to misleading drive line diagnosis. I can't find anyone with a set of ramps.

-- rear brakes I don't know if a rear drum brake problem could be making these noises or not. The differential guy did have to fix my emergency brakes when he put the axle on the jeep.

So my question is this:

Is it possible for a differential to make noises in a specific speed range? Some have advised that a bearing problem would be over all speeds and increase with speed. Some web searches turn up different opinions. When it is making these noises, I can reach back and place my hand on the rear hump, and I don't feel any significant vibration. For the intensity of the rumble sound I would expect significant vibration back there, but I can't feel it. This is the source of my apprehension in immediately condemning the rear end. The noise does go away immediately if I let off the gas pedal in that speed range, which does make me suspect the differential.

It does not seem to change with turns, but again a web search suggests carrier bearings could cause a rumble noise when they are going bad. The guy who installed the differential is baffled as well, and the only thing he can suggest is that maybe the axle bearings got dried out while sitting on the shelf, and maybe a minute rust spot is causing the noise. He did not pull the axles and inspect these bearings.

Could slight tire cupping also cause this? Or even just shitty tires. I would not think it would be affected by load, but the speed range is right for tire issues (plus the slight steering wheel vibration).

I drove from Knoxville to Atlanta and back on the Memorial Day weekend with no problems. At the highway speeds of 60-70 mph the noise is gone and everything seems smooth.

So either take a chance with new tires or start looking for a replacement differential.

Thoughts / suggestions ?

Maybe try different tire pressures? I notice a big difference in the sound and ride of my km2's with as little as 5psi change.

Rotate your tires and see if the sound changes.

Cooper Discoverer H/T's are a pavement tire, and unlikely to cause much noise unless they have serious internal defects. The T-Case chain would make a distinct popping noise, but only when accelerating briskly from stop. The differential is a possibility, but rather unlikely. I would expect the differential to make a “whirring” noise while decelerating, if caused by bad pinion bearings or loose pinion bearing preload, and to make a “howl or whine” during acceleration over a small or large speed range if caused by worn ring and pinion gears or improper gear set up. Drum brakes don't make that kind of noise.

The noises described are most commonly and most likely worn u-joints. Worn u-joints (or an unbalanced drive shaft) often make a low frequency sound/hum at highway speeds, sometimes with a light vibration, that comes and goes kinda like this: hummmmm, hummmmm, hummmmm, hummmmm.

Agree with Tim - check rear driveshaft ujoints and shaft balance.

I recently picked up a low-frequency rumble in my truck coming from the rear end from about 40-45mph and up (became a steady extra drone on the highway), turned out the axle-end joint on the rear shaft had run out of grease and was grinding one of the ears up (half of it was gone when pulled the driveshaft down).