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.

Showing 1 response by czarivey

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.