Power Amp transformer hum - intermittent and increases and decreases in a few seconds

Mechanical hum is not always the result of DC offset. I have some DC offset on my mains and even two DC blockers in series couldn’t eliminate an obnoxious hum in one of my amps.

Anyhow, I own the CMX-2 but found the Audio By Van Alstine Humdinger works far better. I had a bucking transformer that still hummed loud when plugged into the CMX, but it ceased completely with the Humdinger.

Try a cheater plug.

Try a cheater plug.

A cheater plug should only be used to help diagnosis if you have a ground loop problem.  It is not a solution as it poses a safety risk.

Agree.  I did try a cheater plug to no avail.  Thx to all for their input. It is much appreciated. 

This sounds like a mechanical vibration issue. Try this please. When the amp is on and humming lift it from the shelf so it is in you hands not touching the shelf.Is there a change? Next with it on the shelf put your hands on the amp and press on it, like hug it lol, not enough to crash things but to see if pressure on the chassis changes the sound/hum. From your descriptions it is possible the transformer has loosened or in some way is physically vibrating. What brand and model are you listening to? You say it always has had the issue. How old is it? A solution may be as easy as opening the cover and tightening whatever keeps the transformer in place. Then too is could be a defective transformer. GL