Does the grinding happen while turning? I a problem with grinding and had to take it to the dealer about 4 times to get it taken care of. I had metal on metal grinding in reverse and while turning. I finally had to drive the Head Service Tech around in the parking lot until he could hear it. Right after he heard it he said it sounded like a brake problem. It turns out that the pad was moving around a little because the metal pin (or whatever holds the pad down) was loose. Anyway it is finally fixed after 10 months of this crap. I hope this helps.
P.S. I was also once told it was rust.