I have a question about my 2100b. Very good working amp in every way except it has a slight hum when transmittimg ssb or cw around 400 watts. Its wired on a 240 volt 15 amp service just to itself. It gets alittle louder if i increase the power to say 500. Reason I ask is I have a Heath SB220 thats very quite and only hear fan running. I've taken it out and tightened all screws and nuts on transformer. Put a few drops oil in fans. Still has that slight hum or vibration sound when transmitting. Is this normal for this amp to have this sound or am I just comparing it to much to my other amp.