Why do the rear brakes squeak?
 

My fronts were replaced at 42K. The rear pads are fine though the squeak over the past month or so is driving me nuts. My private mechanic took them out and sanded them. That didn't last long. Is there a fix or TSB or something? I have 46K now and just got free motor mounts under warranty. Ideas before the warranty's up? It's only at slower speeds and with light pedal.