Heard it today for the first time, with the amp idle for more than 15-20 minutes. The popping sound comes out at a 2-3bpm rate, but here's the catch: only on high, if the Output switch is set on standby or low the noise disappears. The noise level doesn't depend on the master or channel volume level, gain level or whether there's a guitar plugged in or not. Should I get one preamp and one power tube and start swapping tubes? Or maybe a dodgy solder joint? Other ideas? Thanks in advance.