brakes grind in reverse

When applying the breaks in reverse they will grind, and u can feel the pedal kinda pullsate (sp?) and hear an err-eerrr sound.

now it only does this in reverse and not all the time just sometimes. I tried to get it to do it again but it didnt.

besides going to teh track on weds i washed the car, thats about it lol, my car hates when i wash it

anybody know what this could be ?

thanks
bobby