Sounds to me like there's a humming bird trapped inside! Open that baby up and let the little guy out immediately! That should do it :^)
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I had a similar issue. Since I knew my amplifier's circuit was on the opposing phase/leg at the service panel, I moved that circuit to the same phase/leg (at the service panel) of my other components. As I was curious to see what sonic benefits would result.
I did not notice any sonic benefits, but that coming and going little amplifier transformer hum went away.
That little transformer hum still re-appears but only when the microwave oven is in use.
Not saying you have the same exact problem, but it's worth a shot.
I had a similar problem with my previous Classe CA-150 and Krell KAV-250a amps. I knew the toroidal transformer was humming, but could not figure out why. After much fussing, I finally found that the 125A service panel for my house was defective. It had some loose connections, and parts of it were literally beginning to burn up because so many circuits had been added. I replaced it with a new 200A unit, and have had no problems since, along with a much lower noise floor as well. If you have the ability to do it yourself, it should cost about $300. If you pay a professional, expect about $1700 to $3000. I decided that was just too much money, so I did it myself, but don't try it unless you are absolutely certain you know what you are doing...it can be too dangerous. Good luck.