Power supplies... Phone chargers, ebike battery charger, laptop chargers... So many switching power supplies tend to whistle it's a major pain. Even worse when you're the only one hearing it... Even even worse is that unshakeable feeling that the thing is shit and is going to blow up soon (I played the 'blowing up electronic components' game when a teenager instead of sports or dating or studying).

I can also hear some GPUs around me, and I'm not sure whether it's the draw of the power supply that makes the PC sing or something within the GPU itself.