Near the end of my time living in Seattle, something started making a hum. It was loud, and deep, and penetrating. Miserably loud - and yet my SO, who was visiting the day it started, heard nothing. I went on a trip the next day and it persisted for a while in my head, but stopped the next morning, and I spent a hum-free weekend. Then I came home and it wasn't quite as loud as it originally was, but it was still there. Most annoyingly, it was there in my bedroom at night.
My SO never heard it. I kept on questioning my sanity. Sometimes I could hear it in other quiet parts of town. There were times I'd go into a favorite coffee shop to work and turn right around, because the hum was strong in there. I'd generally stop hearing it if I left my neighborhood.
And every time I'd go on a trip? Nothing. No hum. Even in the middle of the night. But the fact that only I seemed to hear it kept making me wonder - is something wrong with my ears? Or do I just have a range of hearing that extends lower than most people? I got in the habit of wearing earplugs at night and using noise generators to somewhat block it out (hooray for mynoise.net) via a small iPad sitting under my pillow while I slept with the ear that heard it the most, but I still had a lot of trouble sleeping. Sometimes I'd try sleeping in one of the other rooms to get away from it a little.
And then, after about a year or two of misery because I could never get a good night's sleep, I moved to a different city entirely. It's gone. I've almost forgotten what it was like to live with that oppressive, constant, maddening noise that nobody else heard. Every now and then I'm reminded of that and I am super grateful that it's gone.
My suspicion was always that it was something to do with the subway expansion, my apartment was right above the line being dug north from the University District. Some ventilation fan that was making a noise that reverberated through the ground and came up in my ground-floor bedroom. I'll never know.
I did a lot of reading about these sorts of things during this period. Some Hums get found: The sound of a thousand air conditioner fans droning 24-7 to keep a data center cool. A misaligned pump. Fish mating calls. These are all things that a Hum has been verifiably traced to, and stopped. Some Hums never get found.
And it's fucking maddening because everyone you talk to about it is all "maybe it's just tinnitus" even though it stops when you go somewhere else.