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!
It’s almost always the caps (capacitors). They have a very distinctive whine while energizing, and it can also cause some reverb in the speakers. It settles into a subtle whistle when fully energized. 40 some odd years and I can still hear them every time I turn on a computer tower, a TV, an amp, etc. I’ve learned to tune them out most of the time.
Came here to say this. If it sounds like a disposable camera charging its flash or night vision goggles in a video game, it’s probably some capacitor(s).