I was driving at 20 mph on a road near my home when I heard a metallic ping sound then followed by a squealing noise. Long story short, a small rock had lodged itself between the protective plate and the disk. I was able to identify the affected wheel by listening for the squeal noise when rocking the car back and forth. I then jacked up the car verified the affected wheel and spun the tire in the direction as if the car were moving backwards. This process yielded the small rock and the squeal is now gone.