You are hearing what we call in the electrical engineering world 60 cycle hum. Some place in your house there may be a motor or a transformer connected to the same circuit that you are plugging the amps into, such as a cieling fan, fridge, or ballast type light fixtures.
First try using a different outlet on a different circuit. If you still have a problem.......does you amp have a three prong plug? If it doesn't try flipping the ground switch on the back of the amp. Also try using a surge suppression outlet strip with your amps. If none of these fix the problem you may need to call an electrician
and explain the problem. He will most likely check for phase reversal and check for an open ground on the circuit. But plugging into a different outlet may solve everything.