So I take my son's trailblazer to the store last night...as I'm pulling in, apply the brakes and there's a lovely sound like someone dragging a concrete block down the road. Goes away when brakes are off. So I figure time for new pads/rotors. I drove the truck today and brakes are perfectly quiet. It's been 7 years since last brake job, so I'm replacing, but just wondering how they could grind hellishly, then not at all next day?