Purchase some Blu Tack from Walmart, unmount power transformer and place balls of blu tack onto the mounting holes and mount it back on. The noise will cease.
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The strangest thing is- you can only hear it with the amps and the whole system OFF, cold.The transformer is energized whenever the amp is connected to AC power. Turning off the amp apparently disconnects the Power Supply from the secondary of the transformer. Some transformers will exhibit more mechanical noise with no load connected to the secondary than with a load connected.
See note #7 noise
I have a problem with the above theory of disconnected load. There is no practical reason for the amplifier's (main) transformer to remain energized when the amp is turned off. The transformers in some preamps and CD players remain energized 24/7 once plugged in, with only the signal output muted. I have not seen it done with any amp, where the main transformer remains energized all the time while the load is disconnected. Amps that remain in standby when "off" usually employ a separate tiny transformer that is always on, to furnish power to the triggering/soft-start/protection circuitry. Otherwise the main transformer and power supply remain de-energized.
What is your theory?
There is a faint buzz coming from the amplifiers, NOT the speakers, barely audible from a few feet away.