My '08 picked up a pea-sized pebble that got stuck between the rotor backing plate and the rotor. It was actually riding on the inside of the rotor where the vent slots are. It did not damage the rotor, but wow, did it make some noise ! People were staring and pointing when I drove by. Had to remove the brake caliper to get to it. I'm hoping this is a fluke, since I live on a gravel road

 

I think it probably was a fluke! H

 

I also have had this type of occurrence happen to me several times. We get the chip seal events on our roads during the summer which is typically when a rock will lodge itself between the backing plate and rotor. Not a healthy sound it makes for sure. The first time this happened, I pulled over and stopped the car. I then put car in reverse and drove backwards about 20 feet which dislodged the offending rock. All was normal.