Rhythmic sound coming from rear end

Question

There's a rhythmic sound emanating from my rear end, and I'm not sure what exactly it is. It varies with speed and is consistently present, regardless of clutch in/out, acceleration/deceleration, etc.

My rear end is a Dana 44 with 35 inch MT/R tires (4" suspension lift), mounted to a 98 Jeep Wrangler 4.0L. From listening to this sound recording (below), what would you say is the problem? (If I'm not mistaken, this particular noise is something that someone with a trained ear will be able to identify very quickly.)

Recording of the rear end at various speeds: http://s000.tinyupload.com/?file_id=01861567156080059635

Answer 1

There are three things to check. First, check your u-joints to ensure they are in good shape. To do this, chock your tires, put it in neutral, and try to move the drive shaft ends at the yokes. There shouldn't be any play there.

Second, you need to check your pinion angle. Basically, if you drew a line directly back from, and in line with the transmission output shaft, then do the same thing with the differential yoke, these two lines should be parallel. Lots of times when lifts are put on trucks, this angle at the differential gets tweaked and out of line, which can cause vibrations. If the pinion and drive-shaft are in a straight line going to the tranny, you know it's not right. If the tranny is level, so should the diff be level.

Third, you need to check the balance of the drive-line. I think this is the least likely, though.

Answer 2

I've got two strong suspicions. First is the pinion angle Paulster2 calls out - it's common with a shackle lift or SOA lift that the axle gets rolled so the rear driveshaft won't need replacing (too short with the lift). That's hellish on u-joints. if the transfer case end doesn't start with a CV joint, you simply cannot get away with an ordinary u-joint at the axle when the axle's rolled. I personally like to use a torpedo level on the pinion yoke, looking for plumb, when I'm checking pinion angle. Vehicle on a level surface, of course.

Second... is tire noise transmitted inside by a hardened, collapsed, or missing tub cookie. One bad cookie can transmit tire noise from both tires right inside with you. I'd lean more towards this one if the rhythmic wob-wob-wob sound changed significantly on gradual curves left or right, as the treads on the two tires sync with each other, then move out of sync, then sync with each other again - the tires will be rotating at different speeds on any curve.

Answer 3

I think there are a lot of things going on here, so I'm not sure there's a "right" answer (for others who visit this thread). For my particular case, I think it ended up being the following things:

1. Axle bearing going bad (it finally completely destroyed itself on a trail)
2. Wheel noise (perhaps imbalanced, maybe overinflated, etc.)
3. Other things I've yet to identify.

In my particular case - it's a jeep, and I do torture it to a degree... I'm not sure I'll ever find the exact problem. But hopefully something here can help someone else! :)