My '78 Ampeg SVT seems to be picking up and amplifying the sound of the fan in the amp. In "Standby" it doesn't happen. As soon as the amp is on and not in Standby you can hear an amplfied whirl of the fan. If you turn up Channel 2 it even gets louder! I noticed that the amp is picking up other noises too, like when I tap my fingers on the top of the amp it sounds like a drum through the speakers. Is this a microphonic tube maybe?? Any help is appreciated!