Bizarre buzz

I need some theories and hopefully answers.
This is a cituation, neither I, not anyone, who I asked, have encountered before.
It is very simple and puzzling at the same time.
Here it is:
There is a faint buzz coming from the amplifiers, NOT the speakers, barely audible from a few feet away.
Can also hear it with the ear on the transformer cover- sounds like a slight mechanical vibration.
The strangest thing is- you can only hear it with the amps and the whole system OFF, cold.
If the system is playing for some time, and then turned off- there is no buzz.
Only when the system been off overnight.
Disconnect the power cord from the amps- silent (obviously)
Again, only happens whith the amps OFF and the PC connected to the wall.
I'm not naming the brand on purpose, because I think it's completely irrelevant in this instance.
Please, throw in your theories and assumptions.
Thanks in advance.