I had a grinding noise in my LHS, so I checked the front, and the rotors and pads were badly worn. So I purchased and replaced the both front rotors and pads. A few days later the grinding noise returned. Thinking it needed rear rotors and pads, (bought the car used) I pulled both rear wheels, only to find the rotors and pads are like new.

Is there any other reason they would be grinding. It only does it when braking at a slower speed. Braking from faster speeds has no noise.

 

I actually had the caliper bolts strip out, which made the caliper loose. this let it rub on the outer edge of the rotors. You might check that those have not come loose.