I had one of the bolts come loose and back all the way out that hold the caliper to the guide pins. It's pretty f'd up if you ask me. That could cause some serious issues.
Luckily it didn't cause any major damage, just tore off the wheel weights and scuffed up the wheel.
Other thing I just thought of is the pad itself. I had an issue several years back where I got the pad misaligned and when I put everything back together it separated the pad from the backing. What I got was this *ding/ting* sound every time I applied the brake. What had happen was the back got bent back so it was not keeping things tight any more and the pad was moving back and forth.
You may have something similar to that. The pad may not be damaged, but it still may be moving around.