I have had that rock problem too. It was very annoying! I had some mud and small rocks stuck to my tires and as the wheel rotated around, the stuff that was hanging off the side of the tire was scraped off by the UCA bolt. The rocks fell straight down the spindle and squeezed between it and the back of the rotor. The rock then rotated inside of the rotor to the bottom where it was not small enough to squeeze between the rotor and the dust shield. So it just sat there grinding against the inner lip of the rotor and the dust shield. It took me forever to figure this out! Once I found the rock and pried it out the sound was gone. Later I removed the dust shields so it wouldn't happen again.