I've got a Pedulla J2 4 string with a Bartolini preamp. If I fiddle with the knobs (2 band preamps) sometimes I get popping thuddy noise from the bass. Also, if I give the bass a good clunk on the front (near the preamp cavity) with the palm of my hand, I get a similar noise...so it's not the turning of knobs only. Presumably there is something exposed to something in there. Before I open it up, any suggestions on what I'm looking for and what needs to happen to fix it? Thanks!