Turn on your monitor with the PC completely off. My guess is the sound will persist. If it does, you are hearing noise from oscillating circuits which might be causing nearby components to vibrate sympathetically. These tend to be transformers, coils, and capacitors. It's also possible that oscillators above your hearing are producing what's called a "beat frequency". This means they produce a third resonation as they "beat" against each other. There isn't much you can with this. You have very strong electrical fields and charges inside the monitor and such needs to be referred to professionals to correct.