My 2004 xc90 2.5T awd (150K miles) has developed a noise in the rear. I am guessing it might be a wheel hub/bearing, and I am hoping I am right and that it's not the rear differential. What would a failing differential sound like? Would it sound different from a hub/bearing?

If it sounds like you have one snow tire, it is usually the bearing. Drive with the rear windows down to figure out which side the noise comes from.

The rear diff us pretty robust, especially with most of the torque on the front two wheels.
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My name is Klaus and I am a V♂lv♂holic

This might be a good job for one of those non contact IR thermometers,
Not to pricey at Harbor Freight.
Compare temperatures of each rear hub after some highway driving.
I'm guessing that a noisy bearing will generate more heat,
of course so will a stuck brake caliper., etc.
But it might help decide which side is in need of help.

Bill

A bad bearing makes a rumbling or growling noise. Bad gears make a whining sound. But you can't really tell until you can listen to each component since the differential has bearings in it.

If you block up (unless you have a hoist, I prefer cribbing to jackstands) the back of the car to get both wheels off the ground you can listen to the hubs and the differential using a dowel. It will be obvious where the noise is coming from.

Greg

"I prefer cribbing to jackstands) the back of the car to get both wheels off the ground you can listen to the hubs and the differential using a dowel"
That sounds interesting. Would you mind briefly explaining cribbing and the dowel? Cheers

Cribbing is solid wood 4X4 blocks 18" to 24" long built up by laying two N-S followed by two E-W, etc. In this case the top one goes across the pinch weld. I am not a fan of jack stands or ramps and use cribbing to support anything I don't want to be crushed by.

The dowel is held against the part in question and your ear. Any noise will be transmitted along the dowel and be clearly audible.

I suppose I should say that using the engine to turn the rear wheels while they are blocked up is a dumb idea. In this case you would probably need an assistant to turn the rear wheel while you touch the end of the dowel to the hubs and points around the differential.

Greg

In this case, a lift is required as all 4 wheels need to be off the ground. The rear wheels can only be rotated like the car is moving forwards; making the rear wheels go backward tends to lock up the AWD because the drive system expects the front wheels to also go backwards.

I seriously do not expect the rear diff to be at fault.
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My name is Klaus and I am a V♂lv♂holic

Interesting. If you lift both rear wheels off the ground and keep the drive shaft stationary, the differential (AWD?) will not allow one wheel to be turned forward as the other one turns backwards ?

Greg

Not without dropping the driveshaft from the diff. The Haldex gets in the way. Pre Haldex, 1998-2002, the viscous coupling would lock up the propshaft.

The next time Mary's car is up on the lift and in park, try turning the wheels the wrong direction. IIRC, my 1998 would lock even in neutral if only the rear wheels were off the ground.

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My name is Klaus and I am a V♂lv♂holic

Learn something every day. Mary's car is 2WD but I'll try it.

Greg

I had thought Mary's car was an XC70, therefore her car has no propshaft and no rear diff.
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My name is Klaus and I am a V♂lv♂holic