I have had this howling noise or humming noise coming from the rear of my xj since I bought it. It only comes at a certain speeds, when I have my foot on the acceleration to maintain speed. Sometimes the noise is worse.

Today, with almost an empty tank, the noise didn't exist. So what does lacking 20gallons of petrol from the rear affect? What could be making that noise?

 

All that weight missing from the tank may have taken pressure off of something. Just a thought

 

I think that we are on to something Watson

 

Now thats just funny! I would say if you checked to make sure your rear diff has pleanty of lube and the correct lube at that and your tires have correct pressure and your drive shafts have been greased than its probably road noise.

 

If you have the crysler 8.25 it is probably a wheel bearing or ring and pinion wear. Check the fluid level in the diff.

 

Just wondering but what's your reasoning for this ^. I have the chryco 8.25 and have never changed the fluid. Front or rear in the 2 years I've owned it. Which I'm the second owner and was the first the use 4wd. The Po babyied it tho.

 

The axle bearings are known to go bad when you have just a little less oil in the diff.

 

Makes sense. Guess its time for a drain and fill.

 

I better get on mine, I don't think it has ever been done.

 

check your tranny mount. I had unusual sounds coming from my diff's changing the fluid didn't change anything but at a later date when my mount had completely colapsed and was causing vibrations and noise I changed it out and this fixed the sounds from the diff's too. I could see the driveshafts pulling tight as the tranny was bolted back into place. there is quite a bit of hight lost and slack created when the mount collapses.

 

The diff. fluid is fine, and was just changed last fall.

I will have to do some reading on everything you guys have said as I am not familiar with everything unfortunatly.

 

I would start By checking the wheel bearing. Lift the rear and Hold the tire at 12 and 6 with your hands and pull back and forth. If there is play then your wheel bearings are going out. The weight of fuel will create more friction on them making a louder noise. If you feel vibration when stopping that could also be a sign of wheel bearings. Your tires could be getting warped. Good luck!

 

I disagree.
The axial play is normal in both c-clip and 4bolts axles.
The radial play would denote a bad bearing.