I had a pair of McCormack DNA rev a monoblocks years ago that did exactly what you described. It all started when a speaker connection came lose and and was grounded and blew a fuse. I replaced fuse. From then on it made a loud thump when turned on then high pitch noise that went away in a few seconds, but I never heard noise while playing music. I lived with it for a while since I left amp on 24/7. I did end up sending off for repair because I was scared something may go wrong in the future. I was told it had something to do with what they called "the rail".