kinda sounds like my situtaion...

when i back up i hear a metal slapping kinda sounds from the passenger side, and also when i go over bumps. I thought my wheel was going to fall off or something. I noticed today that my caliper is only on by the top bolt and the bottom kinda flies up and slaps back down on the rotor i guess. I have no idea how it happened, but maybe check and see if your calipers are secure. I didnt notice any difference in driving or breaking either, just the sound. it was like that for a few weeks now, would come and go. I never thought to check the caliper until i was walking towards my car today and i noticed the bottom of it wasnt resting on the rotor.