grinding noises

I have a 2000 Grand Am GT with about 34,000 miles on it. It makes a grinding noise when im braking in reverse. Thats the only time it does it. I had the rotors and pads replaced on the front and rear at 28,000 miles because the rotors were badly warped. It made the noise before and after. Ive had it looked at but no problems were found. Any ideas?

Tires, If they are eagles.