Well, some of the tubes are microphonic for starters (typical with 6922 type tubes). Try mixing the (eight) that you have until lightly tapping the "outside" of the chassis does not sound through the speakers. It is not good to tap the tubes themselves, but you may have to try this in order to weed out the bad ones (just do it ever so lightly). When swapping tubes, allow them to cool (maybe 30 minutes or so) before moving them (never touch/move a hot tube as this can damage it). Other than microphonics there may be something else wrong with the amp, but this is where I would start. ALSO BE WARNED that there are enough amps stored in this unit (whether it is on/off unplugged or not) to shorten your career, so if you are not familiar with elctronics consider having a friend or shop (someone informed) take a look @ it. If you use only one hand (keep the other behind your back) and touch only the tubes themselves, you should be OK. If both hands accidentaly conduct a current the electricity travels from one hand to the other (through your heart) which is why one hand is safer. Anyone swapping tubes (a common practice for us all) should be using the "one hand" pricipal @ all times.