Pigeon

98 XJ 4.0 AW4, 231, SUNROOF!, 150K, stock and sagging. 235/75/15 Mud Terrain with >75% tread.

For about 2 months, I've been hearing a sound that I don't like. It sounds a lot like a warped tire, or a tire with a knot on it. It's kind of a faint 'thump, thump, thump'. It sounds as if it is coming from the LF. I don't feel any vibration, and it drives great. The sound frequency increases with vehicle speed. Braking doesn't seem to have an effect.

I have taken it to the tire shop for balance and rotate. They diagnosed the problem as brakes dragging. I have since replaced front rotors, pads, and calipers. This work had no effect on the noise.
I have checked all the front u joints (including the axles), and they appear to be tight.
I have checked the unit bearings by trying to move from 12 o'clock to 6 o'clock, and they are both tight. They also both spin freely, with no abnormal noise. the unit bearings appear to be original to the vehicle.

I have not checked the pinion bearing, nor have I pulled the differential cover. I have not put the entire vehicle on jack stands and ran it.

Can a unit bearing make noise, while still remaining tight, and spinning freely?

Any other ideas?

bad_idea

To answer your question directly, hell if I know.

But reasoning it out, I would say yes. The unit bearing is basically 2 bearings in an assembly. If a bearing gets dry, it will get noisy. Spill a little antifreeze on your tensioner pulley and you will see what I mean. It will chirp and chirp, but it's still good. I would fiddle with the unit bearing and see if you can get some grease into it, and see if it quiets down.

andrewmp6

Honestly if they are that old id replace them.Most have a life of 100k and when they go out you got little to no warning.

Pigeon

I'm going to replace both unit bearings and hope that helps. With 150K on the clock, they are due for replacement. I will at least eliminate them as a source for the noise. 2 Timkens and a new torque wrench are headed my way.

investinwaffles

Timken is great, I went with Precision/National branded bearings and they have been fine for 30k miles now (just FYI)

DFlintstone

Sometimes a wheel bearing noise will change while taking curves at say 40-50 MPH. As the force changes from left to right the load on the bearings changes. Also "normal" worn bearings will run hot, then hotter, then eventually very hot!

Cane

i'm having a similar issue. my bearings are tight but at highway speeds i get a loud rumbling / humming sound. can't hear myself speak to the passenger. i'll be replacing both bearings too and while I'm in there the axle shaft joints too. went with timken and spicer parts

Jeepin'_Aint_EZ

150k on factory unit bearings is impressive. I'd swap em just for preventative maintenance . I'd imagine the lubricant in em don't lubricate no mo'

Pigeon

I'm still trying to find the source of this noise. I replaced the left unit bearing and changed the oil in the front differential today. It made no difference in my noise.

I will change the other bearing when I get time. I am also going to take it back to the tire shop for wheel balancing again. It still sounds like a bad tire. I will likely go to a different tire shop for a second opinion.

investinwaffles

Are you able to hear any grinding in any of the driveline bearings?

It could be a bad u-joint, or centering yoke on the driveshaft, or u-joints in the front axle as well. When mine went bad, I thought my differential pinion bearing was going out because of the "growl",but it actually turned out to be a bad u-joint in the front axle.

Have you put it up in the air and spun the wheels yet?

Pigeon

All the u-joints, including the front axle, appear to be tight. I've checked them by watching them as someone rocked the vehicle. I have also suspected the pinion bearing. I have not put it in the air and spun it yet. That would probably be a good next step.

bad_idea

I would suggest you pull the front driveshaft and take it out for a spin. After that, reinstall front shaft and pull rear shaft. Take it for a spin in 4WD. Should help isolate if it is driveline related and which end it's in.

Pigeon

That will be a good diagnostic practice.

If the front pinion is the culprit, would it still make noise with the front driveshaft removed? I believe that it would, although it may alter the noise.

Cane

it won't make any noise if there's no driveshaft connected to it.

my rear pinion bearing is on its way out and makes noise on deceleration. i removed the rear driveshaft and drove around in 4x4 to do a similar test (therefore making it front wheel drive kinda). the humming/growling sound coming from the pinion dissapeared

mo8414

I had a bad wheel bearing on my jeep when I got it. Would grind more and more the longer you drove it as im sure it was getting hot. Did the usual wheel bearing test to see how bad it was and my tire was solid every time I checked it. So yes a bearing can be bad and still feel good.