At practice tonight, i turned the amp on, left it on standby for about 10 minutes, took it off standby, checked the bias lights as i always do and everything was fine. after about an hour of playing, i heard a very faint clicking/scraping sound coming from the speakers. i checked to make sure all my cables were connected properly, then checked the bias lights and the bias control 2 lights were off. i muted the amp, unplugged the cable from the input jack, and the sound was still there. i reset the bias for bias control 2 and then finished up with practice. the sound isnt constantly there, it just randomly clicks and gives out a scratchy sound, almost like a cable being rotated in a dirty jack or something, but it even happens when the amp is muted and there is no input signal to the amp. the sound doesnt happen when the amp is in standby. when it happens, the lights on bias control 2 flicker a little, and the red light lights up when the sound happens, just like it normally does during regular playing. is this a tube problem? the bias on bias control 2 has drifted a number of times in the past few months. what could this be?