Ask HN: What may be causing high-pitched noise from computer case?

I've got a custom-built Ryzen 5 PC in a Rosewill Micro-ATX case that a roommate built for me a couple of years ago. It has two hard drives and two SSDs, but no video card. Recently it's been producing a very high pitched tone that I can't figure out what it may be caused by. Today I pointed a condenser microphone at it and opened a spectrum analyzer showing the tone is in the 12khz range.

I've turned off speakers and monitors in the past thinking that's where the tone is coming from, but it's from the PC. There is fan noise and hard drive noise that are much lower pitched and easier to identify, but this high pitched tone is very persistent and infuriating and not obviously from fans or drives. What else may it be?