McIntosh hum problem

Are the Mac's on a dedicated circuit? If not you may want to look at whatever else is plugged into that circuit. Some dimmers and motorized devices sometimes can be culprits. Especially suspect would be anything with a high/low switch in it. I had a room humidifier with said high/low switch and it made my velodyne subwoofer amp hum in the low setting.

Try unplugging suspect devices to see if you can locate a source for the hum.

Do they still hum when plugged into another outlet in another room? Have you tried cheater plugs on the units? Is your cable TV near these units?

It is ground noisy, Try use ground lift outlet(amp,D/A)
you can find at home depot or hardware store.
It is gray color , about fifty cents each. -good luck-

Ligi,
I tried different outlets, cheater plugs and no change. Units are in a different room well away from cable TV. Seems to be a problem with noisy power transformers. Thanks to all for your advice.

Those transformers should not be that loud. You need service as a problem prior to the transformer hooup may be creating the noise.