Hey guys, I got the cowbell sound yesterday. This thread addressed 90% of my questions-- I had been driving on some gravel roads recently (don't ask) so it's very likely a pebble got stuck.
From those that solved it: do I just use a screw driver and push in on the dust cover? I assume this is the thin metal element behind the wheel. I didn't think it was mobile and could be pushed in and pulled out, or am I just creating enough space to let the pebble out?
Or, put another way, this thread (11 pages!) had a lot of people that fixed their sounds, but not necessarily detailed directions how =).