The reason I asked about the parking brake is because it is hidden. You cannot inspect it unless the rear rotors are removed. Rust and other gunk likes to build up in the parking brake area and can be a source of grinding noises.

Sometimes while moving slow (10-15 MPH), if you apply the parking brake and listen, you can hear if the grinding noise changes. If it does, the parking brake is a likely source for your noise. Myself...I do this once a month anyway just to help keep the PB areas clean.