2012 iMac still on, even when shut down?

[PavelT]

when i shut down the new 2012 imac. i still hear a electrical current /frequency inside of it. the same frequency sound it makes when it is on. if i unplug it from the wall outlet, it stops of course. it also stops when its been turned off for several hours...also i do have a external usb 3.0 hd plugged in at all times, But when i unplug it, noise is still there...any thoughts? thanks.

note: the electrical buzzing noise is present whether computer is on or off, just as long as it is plugged in.