Transformer hum?

You probably have noisy AC lines, which may be caused by light dimmers. Turn off everything in the house and see if goes away.

Do a search on "transformer hum" for more ideas.

It doesn't matter if the dimmer is on the same circuit. Everything is tied together back at the breaker box. The dimmer on the light in my stove hood upstairs made a balanced line conditioner hum in my basement. If you are in an apartment your neighbor may have a dimmer causing the problem. The fact that a flourescent light makes it worse confirms my suspicion that it is noise on the AC line.

Mechanical hum means the piece is vibrating i.e., the hum is not coming from the speakers. Cheater plugs will have no effect on this hum.

Have you tried turning off everything in the house yet?

Transformers work because the expanding and contracting magnetic fields in the primary windings created from the changing primary currents cut through the secondary windings and induce current flow.

Transformers are designed to operate with a pure sinusoidal waveform. The current swings very smoothly from positive to negative with no abrupt transitions. This means that the magnetic fields are also expanding and contracting very smoothly. If you were to apply a non-sinusoidal wave like a square wave to most power transformers they would hum loudly due to the violent transitions of the current and magnetic fields. Any deviation from a pure sine wave can potentially cause these transformers to hum.

A dimmer works by chopping off part of the AC cycle and this produces noise spikes which travel through the system, and when they get to the transformer they cause it to vibrate. Not as much as a square wave but the principle is the same.