After about 15 minutes of playing my twin will start making a loud sorta buzzing noise. It won't stop until I put it on stadby. When I put it on standby it goes away. Then I can start playing for another 15 minutes or so before it comes back. What is this? Also when I hit some low notes on the E string there is a little bit of a rumbling kinda buzzing noise. I think the twin is a silverface '72. It has apparently been "blackfaced". I got it shipped to me a week ago. I changed out the Ax7's but all the other tubes are what the previous owner had in there. I had a tech. look over and he said he "went over it with a fine tooth comb", but now its making this noise. He said he fixed some speaker wire. He "checked the caps" and did some cleaning of the sockets. He I will say from past experiences I'm not 100% with this tech. Also I changed the speakers that were in there to to California Webers last week. Would that have anything to do with it? If I hooked the wrong speaker wires, what would that do (negative to postive, etc.)? I played a gig with it Friday and Saturday night with no trouble. I have a gig in a couple weeks at a festival opening for Boyz II Men & Kool & The Gang. I am hoping to use this amp but I am scared it won't be reliable.