brake knocking

i have a 96 Lx. i went to back out of my driveway today after the car had been sitting for about a day and a half. as i let off the brake pedal, i heard a metallic knocking sound. i checked to see if anything was loose but it didnt seem like it. the only thing i could see was that one of the anti-squeal plates has slid off the pad a little bit but its still on it. it sounds like something with the caliper hitting on something. when i come back ill take it apart and see whats wrong.

the right side dust sheild is loose. but somehow i dont think thats the whole problem. it only happens in reverse and neutral, and when i let off the brake completely.