Need Thumping Noise Diagnosed

I took my '96 XJ to a highly reputable shop to have 4.56 gears installed in both axles. This shop has been in business for probably 30 years, and ALL they do is gears -- nothing else. They were highly recommended when I first heard of them a little over 20 years ago.

Anyway, I made the one-hour, one-way drive to pick it up tonight after it was finished. The shop's owner (and sole employee) told me that my passenger side u-joint on the front D30 axle was bad so he replaced it. He said that the other axle u-joint and bearings looked good. He also said that the front and rear driveshaft u-joints were fine as well, although he did mention that he had a hard time with the front driveshaft (like it was too long or something and barely fit.) He had a hard time removing and replacing it.

Well, I started home in it, and immediately heard a thumping noise coming from the front (driver's side perhaps). The thump speeds up as I give it the gas, and slows down as I coast to a stop. It is definitely something rotational in the front. I immediately turn around and return to the shop. We lift each front tire separately, but don't hear the sound when rotating the tires by hand. I take him for a test drive, but since he suffers from complete hearing loss in one ear, he could not really hear it. I then let him drive, and he still couldn't hear it. I myself could hear it plain as day - thump, thump, thump...

After puzzling over it for awhile, I finally told him that I'd try to figure it out later on my own. I drove the one hour home, and let me tell you, my knuckles were probably white and sweat was probably beading on my forehead as I drove 60 MPH all the way home in the dark. Luckily, nothing snapped, and no wreck occurred.

So, the big question is, what the heck could it be? Could it be the u-joint on the axle end that he didn't replace? Could the front drive shaft make that noise? I'm about ready to pull my hair out, and I have a migraine headache right now as I type that few could imagine.

If the thumping is a high frequency then it is related to the driveshaft. If it is low frequency then it is related to the axles or unit bearings or something. Lift the front wheel off the ground and check for up and down play in the unit bearing. While the front is up, rotate the wheels one at a time by hand and listen/feel for any irregularities.

make sure the lug nuts are tights too.

The only way I can describe the sound is a thump, like something is loose and flopping around. Everything checked tight. When we had each front wheel lifted up, we checked for top-to-bottom play and found none. There was a slight amount of side-to-side play. Spinning the wheels by hand produced no abnormal sounds, nor did we feel any vibrations through the wheels. The sound is definitely rotational and not created by hitting bumps in the road.

All 4 wheels had tight lug nuts.

I might will want to check the driveshafts or even get either front or rear out to rule out or pin point the driveshaft(s) being the problem

double check on the u joints. I had the same issue but the joints felt okay. Took it all apart and replaced joints. Problem solved. They may feel good and tight when not moving but when crap starts moving it may play a diff tune.

What's odd is that the noise wasn't there before I took it to the shop. The guy said that it's quite possible when taking stuff apart and putting it back together that something may have been bad but didn't show symptoms until reassembled, and all it took was a little messing around with it to finish it off.

I guess I should pull the front driveshaft to see if the noise persists. With as much trouble as the guy had getting it out and back in, it may not be easy.

yeah try pulling the DS and see what happenes. If he had so much trouble getting that shaft back on he may have jacked up the pins/bearings in the ujoints. You have to be gentle putting those back on.

check to see if the front driveshaft (and/or rear depending on the year) is correctly timed.

if the front and rear yokes on the driveshaft aren't lined up correctly, it will give you this sound..and will kill ujoints quickly

Based on the original post, I'd suspect the front d/s...take it completely out and see if the noise is still there

I got to thinking about it, and I'm inclined to believe that it has something to do with the hub area instead of the driveshaft. My reasoning is that the timing of the sound corresponds to the speed of the tire rotating. The driveshaft should be turning around 4 1/2 times faster than the wheel, which would result in a faster frequency of noise, which doesn't seem to be the case.

M4Madness said:

With as much trouble as the guy had getting it out and back in, it may not be easy.

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This indicates to me that this might be the issue.

Try swapping the unit bearings side to side and see if the noise moves.

jeeperjohn said:

This indicates to me that this might be the issue.

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He said that it was difficult to remove due to its length -- that he had very little room to get it to clear. It is the factory shaft, so I guess it has something to do with the RE 3.5" kit and .75" spacers affecting the distance between the pinion and transfer case.

If I knew that it was the hub units for sure, I'd just order a couple of Timkens and be done with it. With 160,000 miles on the original ones, it's probably time anyway. We never felt any top-to-bottom play in them. One thing's for sure, they should be easy to get off since they were off yesterday and he used anti-seize when reinstalling them.

he may have hammered on the unit bearings to get them out and caused damage, granted it seems he's been around for awhile and would know not to do that, I would look into the axle joints and make sure they are good even though one was just replaced you never know......