When Marty is hiding in the dropped ceiling, and the big fat guy goes down the hallway that he's in, he shoots the shotgun, cocks it. Shoots the shotgun, cocks it. Shoots the shotgun, cocks it. Then, Cosmo tells him to stop shooting. He talks to Marty over the intercom for a few minutes, then Marty opens up the tile in the dropped ceiling. The fat guy then cocks the shotgun AGAIN, and we can hear the obviously empty shell rattle around on the floor, because there is no other noise.

Dramatic effect strikes again!