Unknown whump in the XJ

Stock 96 XJ here and the old girl's had this problem ever since I bought her 11 years ago. There's a whump sound that matches exactly the rate of tire rotation and I have no idea what's causing it.

Here's the facts ma'am...

- It's been happening since before I got this recent set of tires so I know it's not an unbalanced tire.

- It seems to vary somewhat but is always present

- At speeds under 10-15 you can sometimes feel it in the seat with the truck body bobbing slightly up and down in rhythm with the sound

- Does not translate to the steering wheel

- I just got all the wheels off the ground w/ jack stands, first just the rear then all 4, had it in both 2 wheel and 4, gassed it to get the tires spinning and didn't hear or feel any whumping at all.

My buddy that I bought her from and a couple other yahoos loaded her down and went tearing through a field one fourth of July, you could imagine, and ended up bottoming it out on a pile of cut logs. Best I can recall, and by that I mean not being able to recall anything at all, I think those logs we had to yank out of there were jacked up under the drive shaft. I thought that might have something to do with it, but talking with him recently he says he didn't notice any problems with it the next day so now I really don't know.

Any ideas what this could be or maybe what I can do to start testing/examining parts for the problem?

have you done any rear brakes on it? could be a out of round drum. or possibly a axle ujoint or wheel bearing.

driveshafts spin faster than the wheel so im not sure if it would make a matching noise.

Dang.., perhaps the whole vehicle is sort of 'banana' shaped as per landing on a pile of logs. Could it be that the whole drive-train-line also takes on that curved line, i.e., not straight. Such a banana shaped line could so subtly cause a fractional binding, i.e., all combined components together throughout that bent banana line are cycling together something like kids playing/swinging a jump rope!?! Like you have described feeling a bobbing sensation meaning an up, and down feeling in your seat.

I'm thinking here that I am stumped, (play on words), as I've yet to land on a pile of logs, (or a stump). One kid in a car did that in my area, missed a turn, went airborne, landed, and thusly totaled with the front, and rear of the car lower than the middle.

Try checking your line, (of the entire drive-train-line), you might find the engine might need to be relieved, i.e., loosen the motor mounts, and then re-check that line angle all the way back to the rear differential. Or not, meaning forgo the motor mounts, and check rearwards anyways. Hardware stores often have an angle finder, magnetic attaching triangle thing, with a swinging arrow that will point to some embossed degree marks. Can't tell you what angle that is supposed to be, but you will want every component to be along a similar angle so as all are on the same vector, or think line of sight angle.

Example, when you give the XJ a lift, it is often recommended that you relieve the motor mounts, and check that newer steeper line to be within limits of being a straight line. Company I bought my lift parts from really pushed the idea of a "T'-Case lowering kit, (easy peasy), in order to to ensure that straight line of the whole drive train. The motor mounts are relieved to install kit parts, and then re-tightened upon completion of installation. Chalk, and a cheap key-chain laser pointing device might prove useful too.

Anyway, it won't hurt to insure the entire drive-train-line is true. At the very least it will be one variable either out of the picture, or not.

Another thought is to ask an alignment shop if they can perform those checks as they have all kinds of neat ways to check things.

You took out the tire problem often causing such hopping harmonics, but did you change out wheels too? If a wheel is out of round, it can cause the same hop, with old, or new tires.

I would inspect the motor and trans mounts. Inspecting the trans mount really requires that you drop the crossmember. The two halves of that mount can be totally separated and yet still look fine for all that you can see with the crossmember still in place.

Pull each wheel and check the brakes, looking for anything that looks irregular, especially any signs of a caliper on one side moving back and forth more than its counterpart, or a pair of shoes moving back and forth.

You can try to get a feel for the wheel bearings, but I don't know that an unloaded test will accurately represent what you are feeling when under load.

I would certainly pull both driveshafts and check each U-joint.

Let us know when you finally figure it out.

Thank you guys for all the good suggestions here.

One thing I can rule out is rear brake drums. Changed those out a couple years ago and it had the problem before then.

I'll start working on all these suggestions.

Question though.. I'm afeared it's a bent axle tube. What's the chances of that?

Doesn't sound like a bent tube. Why don't you take a video with sound to better understand what's really going on?

Check the rear axle wheel bearings for up and down movement. Bad bearings may only show up when they have weight on them and not when the wheels are off the ground free wheeling.

How many miles?
Front d/so double cardan joint maybe.
Axle carrier runout too excessive.