I have been loving my new-to-me Super Champ XD, but while I was playing it a couple nights ago with my Tele, from out of nowhere it just started making a very loud crackling sound, like a geiger counter. Volume knob didn't change volume of sound, makes sound with or without guitar cable plugged in. The only things I could find online about this sound suggested it was a cell phone or some kind of interference, like a florescent light. It's not either of these things. It's very loud. I have only turned the amp on one other time since to record the sound, because I was worried I might cause further damage. Any ideas? It has fairly new JJ tubes with low milage. I tried to attach an mp3 file but forum doesn't seem to allow it. Thanks!