My Mod50 has begun making a crackling, static-y sound. The problem happens when the amp is warmed up, and it stops briefly when I tap the top or side of the head with moderate force. At that point the amp will be quiet for maybe 3-4 minutes, then the problem starts again. It happens whether I'm playing or not, and is present at all volume levels. So far I've checked to make sure the tubes are seated properly, but have not done any other troubleshooting. Ideas as to what could be causing this?