I had something along these lines happen to me years ago. It was sound-related, but didn't turn out to be electronic. I was renting in a century-old house that

I had something along these lines happen to me years ago. It was sound-related, but didn't turn out to be electronic. I was renting in a century-old house that had been converted into apartments. Every night, around midnight, this very high-pitched hum would start, and continue until almost dawn. None of the other residents ever heard it when they were around. While the exact timing varied, the time during which it happened overlapped with my sleep schedule (I was working a late tech support shift at the time) and it kept me awake. Earplugs didn't really seem to help. There didn't seem to be any obvious source. I did exhaustive work to try and track it down - even got the permission of the owner and my fellow tenants to try shutting off the power at the breaker box in the middle of the night to see if that helped. (It didn't.) It wasn't always constant, but it was around enough and preventing me from sleeping enough that I was going slowly insane. I had my ears checked for tinnitus - nope, I had and still have darn good hearing. I started reading about the Taos hum, looking into resonant frequencies, and finally stumbled across the answer. The noise started a little while after everyone in the building went to bed. It stopped shortly after they got up in the morning. It was the sound of the century-old plumbing system at full pressure. If I just left a tap open a trickle, and the noise went away along with my sleep-deprivation.