A lot of 'popping/static' sounds are caussed by poor pin contact. To begin your search I would first eliminate this possibility by cleaning the pins. If it continues thereafter, I would switch tubes from channel to channel and see if the sounds change channels as well. In fact I think I would do this first as often just the removal and replacement of tubes is enough to clean the pins sufficiently for good contact.
If after switching tubes (and thereby eliminating the possibility of a bad tube) and cleaning pins you are still having noise problems in the same channel it could well be poor contact in the tube sockets themselves. You can correct this by simple re-tensioning of the contacts in the sockets.
If after switching tubes (and thereby eliminating the possibility of a bad tube) and cleaning pins you are still having noise problems in the same channel it could well be poor contact in the tube sockets themselves. You can correct this by simple re-tensioning of the contacts in the sockets.