I've got an amp and am experiencing a strange noise once it gets hot. It's hard to describe, but it sounds like a buzzing noise, it is only produced when the a note is played on the guitar, and it sounds like it's an octave higher (it's hard to tell exactly) than the note being played. The sound does not occur when the amp is still relatively cold. I've tried switching tubes around with no luck. Could this just be a bad pre tube?