I have a 5E3, with Pro Transformer, running 6L6s, bias adjusted properly. I was experiencing some noise (crackling) that seem to go away with a tap or wiggle on the preamp tube (12AY7). Heavy pick attack on bass notes started a "machine gun" type noise (mechanical sounding 'tack-tack-tack-tack') that was very loud and didn't stop with muting strings. I turned the amp volume down, then back up it went away. Got it to start again, wiggled the same tube and it seemed to affect it. I changed tubes (12AT7 this time, only one 12AY7 on hand). Played the amp for another hour, no problems. Later, went back, played really hard again and it did it with the new tube, not as bad, but still there. Seems to be generated with heavy playing on bass notes with very bassy eq dialed in. Lighter playing doesn't affect it. It does it with channels jumpered or not., I can play forever with great tone as long as I don't push it hard with alot of bassiness (is that a word?). "machine gun" doesn't really describe it well... kind of a crisp metallic "motorboating" perhaps? No other issues that I know of... I have a third tube in it now, another 12AT7, and it hasn't done it for a day or two... could it have been an issue with both tubes or the interaction of those tubes with the rest of the circuit? I have since used both the tubes in another amp with no problems. Any ideas???