I changed out both inner and outer hub bearings on the drivers (broken axle) side today. No change in sound. Moved tires around. Same same.

The sound definitely changes with brake and gas. But it's never completely gone. Unless stopped. Same sound and frequency when gliding in neutral. The more speed the higher the pitch. After about 40mph I can't hear it until I slow down. I'm thinking pinion bearings for some reason. Ugh.

 

Can you lock the center diff and drive it with each of the driveshafts off? That might help zone in on what it isn't. Maybe a stupid idea..

I had a pinion bearing go out on the back diff of a Jeep ZJ (Grand Cherokee) this summer. One bearing broke and they all tilted sideways. It vibrated like CRAZY when I drove it home at less than 25 mph. But even when I had it up on jackstands I couldn't tell the pinion was toast until I had the wheels off and turned the yoke by hand.

But you said you had the carrier out and was rotating it by hand, right?

I once had a rock get wedged between the fins of a rotor and made a strange noise, but Discos are solid rotors ...so, I'm kind of stumped. Does the outer edge of the rotors look good (like they aren't rubbing against anything).

Did you check the u-joints by hand? I can't remember if you say you replaced them or not..

 

Can't lock the center diff. Its a Borg Warner case.

Yea when I had the differential out I checked for excessive movement of the pinion and carrier bearings. I guess that doesn't mean squat. I don't have micrometers installed in my eyeballs. Usually I can spot a bad bearing. Either by feel or by sound. Or visible damage.

I just find it hard to believe that the pinion bearing or the other bearings could be that bad with out some sort of sign.

I've looked at exploded views of Detroit's and how they work and I can't understand how they would make noise like this. Even if it was not functional. All it does is engage and disengage. So even if it was stuck or cracked or in some way comprised....it would still have to be the differential bearings making a rotational hum. It only rotates on the two main carrier bearings not internally Right?

I replaced the front u joints but not the rear. They are tight and I just greased them. I'd have a hard time believing a u joint could make this noise. I guess crazier things happen

 

Have you put the up on jackstands and ran it in gear (watch out for spinning wheels) and gone around with a dowel pressed against your ear to see if you can zero in where the noise is coming from? Have a partner in the drivers seat run the gas for you and you 'carefully' go around the vehicle pressing on the main bearing areas. Of course you won't be able to check the driveshafts. Have you pulled the rear driveshaft and felt the U-joints to verify they move freely? I'm with you, it sounds more like gear noise, but got to rule out the easy/inexpensive things first.

I'm guessing it's in the gear setup. I would put the carrier on an engine stand and paint the teeth and check the pattern. Maybe they used an aftermarket gearset too, which can sometimes be noisier than OEM. Or, it could be something else too...

watch this video:

 

Did you put it up on jackstands, remove the wheels, and turn the hubs by hand to try to replicate the sound?

 

I did but I found I couldn't move the wheels fast enough to make any noise. I would like to try and run it w wheels off. Makes me real nervous to run it with the wheels on. It would be a disaster if it tipped and took off out of the garage. I've done it on a lift but never w an AWD vehicle on jackstands.

 

You know even with the BW case you can drop a shaft and drive just fine. Just bring something to block the tires with as it will roll away when parked. I learned the last part the hard way, luckily I had a nice neighbor let me know my RRC was running away.

 

I'm pretty sure that only works if your viscous coupling is bad/locked up.

 

Nope works just fine with a good VC it spins and locks. You just get a slight hesitation off the line.

 

I wound up replacing the two main carrier bearings as well and it has quieted down a lot. I suspect that the pinion bearings may be to blame but for now I'm saying screw it. Now I can barely hear it. If it gets noisier I'll address it. The bearings I pulled out looked and felt perfect. I dunno what the deal was.

Doing those two wasn't that bad. Adjusting the preload on these isn't as bad as like a Chevy rear.