chirping/chattering from rear end
 

This is on an S4 TII drivetrain, when im driving I'll hear a high-pitched chattering sound from the rear end/wheels and feel a slight vibration through the car. It happens when going straight or turning, and i've only noticed it on the highway. It never seems to get very bad, and if i have the radio on at all it will pretty much drown it out, but I was wondering about the cause. It happens in over-run, cruising, and very light throttle, but seems to go away when accelerating. I know that I warped the front brake rotors recently, and was thinking that I may have done the same to the rear (even though it was always the front locking up first; controlled braking tests after replacing the fluid & new rear rotors). Could this be the wheel bearings, or is that a different sort of sound?