Intermittent noises can sometimes be a bad power tube. Hae you tested them?
BTW I have this amp and I have not had this problem, but I was having intermitent noise issues with my Onix SP-3 and it was a bad Valve Art 350B tube.
BTW I have this amp and I have not had this problem, but I was having intermitent noise issues with my Onix SP-3 and it was a bad Valve Art 350B tube.