A high-frequency whine coming from my LCD monitor. It makes this noise even when it's on standby. It also produces a loud 50hz mains-frequency buzz when I turn the brightness down. Also a slight hissing from the charger for my toothbrush and the rumble-whooshing of traffic outside.
On the plus side, I no longer hear so much as a whisper from the rest of my computer, since I put it in an adjacent room and made a hole in the wall for cables to go through, and also have quieter components in it now.
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