You'd be better off having a tech to listen to it in person. They'd usually do it at no charge, plus that first hand account would go much further that me trying to guess what the sound is like or what area underneath it comes from. (Unless, of course you could upload a video of the car clearly making the sound, that would be nice.)
August, 4, 2014 AT 5:36 PM
I don't have a video but I have some more details. Noise stops when belts are disconnected but returns when the serpentine belt is reconnected. The noise only sounds when in reverse and drive gears (no neutral nor park).
August, 5, 2014 AT 6:25 PM
Check the belt tensioner pulley for wobble or any indication that the bearings are about to fail.
With the belt off you can check each pulley for noise when you spin them by hand (this should help you locate the problem).
By the way, Is the a/c on when you hear this noise? If so, try it with the a/c off to see if it goes away.