I usually don’t unplug my speakers (active studio monitors Adam A5X) from the power outlet, but when I do, there is temporarily - usually for a few minutes - a very high pitched whining, almost signal like, noise coming from the tweeters when I reconnect them to the power outlet and turn them on. I don’t know much about electricity, but if I may make a wild guess: could it be the noise/subharmonics of some capacitors being “refilled” after being completely discharged upon being disconnected from the power outlet? But then again, wouldn’t that happen in a few seconds?
I am very eager to learn!
I’d imagine it’s the electronics before the amplification stage being energised and the resulting noise is being amplified and sent to the speakers. Or it could even be the amplifier itself powering on and letting noise through. I don’t know much about the specifics beyond that, though.