Whining from rear end
 

Hi all, 2011 Saab 9-3 XWD, currently 9k miles. Just purchased a month ago. Was previously a dealer loaner vehicle.

I have this noise coming from both sides of the rear when the car is in motion. I am unsure if it is just loud enough to sound like it is coming from both sides or if it is really coming from both sides.

It doesn't start immediately, only after the car has been driven a bit. The noise is most noticeable between 5 mph and 10 mph. It is definitely a rotational sound (I can notice fluctuations in the noise as the wheel spins). It appears to come from both sides. When the car is in the air and the wheels are spinning their is no noise (according to the shop I have been to, I have not been present when this has been done), so that would mean it only occurs when the wheels are touching the road. It does not change when the brakes are applied but goes away immediately when the car comes to a stop. It occurs no matter which gear the car is in (automatic). It even makes the sound when the car is off but rolling.

I have taken it to the dealership and several other repair shops. They all take apart the brakes and inspect the drive train, but no one can find anything out of the ordinary.

I am at a loss, my last avenue of investigation is the tires, which will have to wait till summer when I get new tires.
Could cupped tires cause a whining type noise? Should I be worried about the differential (diff whine?) Could it be a sound caused between friction between the drive axle and its boots (either at the hub or the differential end?)

I also have not had the hubs pulled to inspect the wheel bearings. But I replaced several wheel bearings on my 9-2x and am familiar with that sound and symptoms. This noise doesn't seem to match what I have experienced in regard to wheel bearings.