What is this high pitched whining noise temporarily coming from active studio monitors after reconnecting power?

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!