One possible cause - if you get any direct current on the circuit, it may cause the transformers to "hum," or "buzz" if you prefer. My RM-200 had a fairly audible intermittent hum. Like you, it was a noise from the transformers themselves, and not anything you could hear through the speakers - they were dead silent. Not a hum, hiss, or noise of any kind. I finally isolated the cause to a lamp with a dimmer on it, which in one setting would make the transformer hum, and in another it didn't. The lamp wasn't even in the same room, but was on the same circuit. I moved it off the circuit and cured the problem.
Unplug EVERYTHING on the circuit except the amp, and see if you still get the hum. Then you can add your other electronics one at a time and see when (if) it reappears and you have your culprit. Someone else on the forum may have an easier way to find DC on the line ??? Any DC on the circuit will cause most toroid transformers to hum like crazy...
Good luck!
Mark
P.S. Roger may at one point have wound all his transformers himself, but no more. You pay a premium for that, I think about $1K. I've never heard them, but am considering the option for my RM-200, unless I decide to bi-amp (which is more likely) my high efficiency speakers.