My daughter's car's front disk brakes grind loudly, only when stopping. The noise is a low rumble, not a squeal. Roadside visual inspection through the spokes of the wheel showed a smooth, mirror-like non-grooved rotor on both passenger and driver sides.
After sleeping on it, I now wonder if perhaps a pebble is lodged in the kerf of one of the inboard pads, grinding at the inboard surface of the rotor. I'll check it this weekend.
But, I'm wondering how serious that could be. Under what circumstances could we just pry the pebble out, reassemble, and drive on? And at what point would we be forced to replace the rotor? They are slotted & drilled, so I'm not sure resurfacing is an option. (They are less than 2 years old.)
If a rotor has been grooved by a rock stuck in the pad, then the pad will have a complementary groove that mates the rotor perfectly. As long as the rotor isn't gouged so deeply that it is out-of-spec, then I'm hoping the answer is that we can save the expense. (Although, safety is the most important thing.)
UPDATE: no rock grooves... both rotors look brand new on both sides. Still hunting down the source of my noise, however I still think this question has value for future visitors (if someone cares to answer it for the general case.)