Hey All, I recently purchased a used Supro Saturn 1648RT. Played fine in store and for like 2 weeks at home. Then it randomly started making this noise (see video below). Like a buzz that decays with the sustain of the notes. Most recently the volume had started to dip and then come back as well. It doesn't sound vibrational to me either and I can't find anything loose. I have replaced the power tubes and swapped out preamp tubes as well. (the ones that are accessible from the rear of the amp) I have plugged the amp into a different speaker as well. I have tried every guitar/cable/pedal/no pedal combo. Still it persists. It is important to note it comes and goes but doesn't seem related to the time at which i am playing (right after turning on, or when its been running for an hour +) I haven't opened up the chassis yet either. Thanks in advance for any help!