When I step on the brakes the car is making a grinding noise as it slows. I was sure the pads were worn and there was metal to metal contact except that it doesn't always make the noise. It seems to be coming from the rear, although it is hard to localize, and it is more noticeable at slower speeds... right up to the point of stopping. The frequency decreases as the car slows so it must be tied to the drivetrain/wheels. The sound sort of reminds me of a truck that is engine-braking, but not nearly that loud. I have pulled apart all four brakes and the pads are fine and everything is moving smoothly. I also pulled the rear rotors (there is some wear on them) and checked the e-brake pads. What could be making this noise?