My sentra did this and it was because the rotors were worn to hell. I needed new ones. However, the maxima sometimes does this (for a while it was doing it quite a bit, but has since let up). I've checked the brakes and had the pads in my hands and confirmed that they, and the rotors, are in great shape. I still don't know why it grinds sometimes, but seems to prefer doing it after some heavy braking or semi-performance driving. The stock 00 maxima rotors are notoriously bad, so I guess they could be at fault, though I still don't know what is actually causing the grinding.