Weird noise from the rear end

Mine does the same thing. Best guess is it's the clutches slipping and reengaging in the cold differential and you're hearing the splines in the axles take up the slack moving from loaded to unloaded tension on the axle shafts. I've had the brakes and calipers off several times and wire brushed the hub and rotor mating surfaces and checked everything I could think of. 72k miles on the car now and everything seems to be just fine, except I still hear that little annoying noise every now and then.

I'm beginning to think there aren't any real mechanics left at dealerships any more. I hear the most bizarre statements made by obvious morons, and we are supposed to trust their diagnostic ability with the most expensive thing we own! If the computer doesn't give them a code, they don't have a clue what to do.