i think it might be the rear caliper. The pads could be rattling in the caliper. is there a clip taht holds in teh caliper?

 

i have the same problem have no clue what the hell it is, but its starting to bug me. once i get home this friday im going to tear in there, and wiggle things around, im assuming it might actually be the caliper, because those rear brakes havent been changed in ages.

 

heat shield mate and very common problem

 

Yes, there should be a metal clip on the caliper, you can easily see if this is missing or not, sounds like a good posibillity.