ALWAYS replace/resurface rotors when you change pads. This is the reason why and people get pissed until they don't complain about noise. When you put new pads on new rotors they essentially mate with each other, then if you "pad slap" and put different pads on than the last set you are changing the materials....period. Each company's pad material is a little different, thus causing noise.
My theory at least.
OR, if you did take care of the rotors (which would be important information in diagnosing the issue) it could be one of the clips on the caliper brackets where the pads ride, OR a backing plate.