Incubus If your guitar was plugged in then it is probably due to lack of shielding of the electronics cavity, and potting of the pickups or a grounding issue (disconnected ground somewhere in your guitar/cable/amp/building.) or oxidation of a connection somewhere (like your guitar in jack.) If it happens even with the guitar unplugged, and there isn't a grounding issue, one fix might be to add a ferrite bead/choke to your power cable, where it goes into the amp also try cleaning the jack or getting a new cable.