The rock between the dust shield and brake rotor happened to me last year in Moab, and VehiGAZ is right; it sounds like cat being goosed with a hot poker. It's a HORRIBLE sound and it happens constantly, so I don't think that's your problem. Have you checked the bearings lately?
"The major difference between a thing that might go wrong and a thing that cannot possibly go wrong is that when a thing that cannot possibly go wrong goes wrong, it usually turns out to be impossible to get at or repair."
-Douglas Adams, Mostly Harmless