The pads absolutely cannot rattle in the calipers. If the caliper bolts are properly tightened, they absolutely cannot rattle either. If its a clanging type of rattle, its the backing plate, easy to bend and just as easy to bend back.
As I understand there is a very small gap between Brake Pad and the Rotors when the brakes are not applied. So, there could also be a very small gap between the Brake Pad & the Caliper Piston