4WD and I’m pretty sure its the rear differential.
Long story short, Mrs Cad put about 100 miles on her jeep after it started making a weird sound and slight burning smell so she could bring it home for me to look at (IANAM). She thought it was the wheel or tire. I drove it a few miles to the store tosee what was going on. Accelerating it makes a whine like a model airplane. If I let off the gas, it makes a grinding sound and sometimes will vibrate a little. Is it possible it can be a wheel bearing? I’m pretty sure its not the brakes.
Does it make any difference if 4wd is engaged? Knowing the model might help, depending on whether the car runs in fwd or rwd normally it may make a difference when you engage 4wd.
If it was a wheel bearing it would get worse as you turn away from the affected side (due to weight being transferred to the bad bearing) but these tend to make a rumbling sound, not a whine.
Have you had a look underneath? Check for oil on, around or dripping from the diff, and just have a general poke about, look around the wheels, halfshafts and suspension for anything hanging down, it’s possible you have something dragging on a rotating part, it’s always worth giving it a good coat of looking at.
It sounds more like the differential, but a wheel bearing can make that noise as well.
So drive down a quiet street about 35MPH and swerve from side to side. If the noise changes dramatically in one direction, the wheel bearing is highly suspected. You could carefully jack up the back end (both sides) and run the vehicle while listening for the noise. Also, if you drive the vehicle a few miles with the noise and it’s a wheel bearing, you should feel a difference in the heat when you CAREFULLY place your hand on the center of the rim. If one side is much hotter, that’s the bad side. And it really could be brakes too because that would provide more smell than a bearing or differential.
I checked the heat of the rotors after a few miles and they were the same temp so I don’t think it’s a brake problem. There is a side road next to the house so when I get home I’ll do some safe swerving. I saw some oil on the crankcase but not the differential. Could burning gear oil or anything in the differential make a slight burning smell noticable from the outside?
U-joints would have been my first guess but they don’t smell, an axle going out supposedly whines but I’ve never heard of them having a burning smell. Of course, run them dry and who knows. Are you familiar with what used gear oil smells like? To someone who isn’t used to it, I’d say that gear oil itself smells like a burning skunk.
I’d check the exhaust system and see if anything is spilling/splashing on it. It could be something that wouldn’t normally smell getting on the pipe and getting hot.
Does it make any strange sounds while sitting still or only while moving?