Tubes with long plates tend to be more microphonic. With the system on and at idle you can tap on each tube with a pencil or you can tap on each tube with your fingernail to locate the microphonic tube or tubes. I have heard of microphonic output tubes, so be sure to check all tubes.
Ringing Sound In Tube Gear
I recently have begun noticing a feint "ringing" sound while playing my Onix H34 integrated. It comes and goes and I'm sure it's not my ears. Could this be a problem with the output tubes? Any advice would be helpful. Thanks