I just broke down and called Outlaw and talked to Steve. He said that it is most likely a low voltage or low current situation that's causing the transformer to vibrate intermittently.

Seems that if there is something on the same electrical circuit that pulls a lot of amps, the M200 is sensitive enough to react this way. This could be anything from a space heter to a powerful subwoofer...

He also pointed to a powerstrip that could be 'constricting' the flow of power to the units. Over time, each time the strip takes a surge, it slowly wears out. Since the M200s are electrically and thermally protected, he said there isn't a need for the surge protector anyway. A power conditioner or a regulator, perhaps.

My first course of action will be to look at all the items on that branch curcuit and see if turning any devices on causes the hummm. Second, if that fails, is to isolate the amp on a different circuit with an extension cord and see if that stops it.

Wish me luck.
Rob