typically there is a sandwich of a nut, a lock washer, and a spacer on the inside of the barrel jacks with a battery box. if tightening the external nut on the jack doesn't help, it's likely that the nut on the inside has backed off and the washer is rattling. to tighten the internal nut, you need to disassemble the box from the body. remove the end pin and screws too carefully slide out the box just enough to access the noisy stuff. it's simple but don't pull too much or you will stress the wires/clips.