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?

 

My guess would be a bent wheel

 

That was my first reaction as well. Are you running steel or alloy wheels?

 

A bent wheel should have been spotted by the tire shop, but maybe not. A bent axle, which is what I'm guessing it has, they wouldn't have noticed. When you had it on stands & the tires spinning did you get out & look or just sit in the jeep? If you didn't get out & look that's what you need to do.

If it were the driveshaft the speed of the sound would be much faster than the tire speed, 3.55:1 being the most common.

 

I believe they're alloy wheels but not entire sure. They're the 5 thick spoke wheels that come in either lighter brushed steel or a light gray color.

Little confused about something though...concerning it being an out of balance wheel. When a wheel balance is done, would it matter whether it was the wheel or tire that was causing the out of balance state? Once a weight is put on I would think the entire thing as a unit would be balanced.

Yeah '90 looking back I should have got out and had a close look at the wheels. Next step is to get it off the ground again and do that.

On the bent axle idea... now would that be a bent axle shaft where I could just put a new one in, or are we possibly talking about a bent tube in which case I'd probably have to replace the whole axle?

 

A bent wheel and an out of balance wheel are two different things.

 

Yep, this is what it sounds like, and it doesn't take much bend at the axle to make a huge difference at the tire/road contact patch.

Your point about the ratio difference in driveline sounds/vibrations and wheel sounds/vibrations is something very important to help judge this stuff!