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