A couple, or maybe a few, years ago I began hearing a low hum, kind of like a pulsating bass note in my bedroom when I would lie down to go to sleep. It became quite annoying and would make it difficult to fall asleep, and would sometimes wake me out of sleep. This went one for weeks or months and suddenly stopped as abruptly as it had begun.

Months went by and then 'the hum' returned. I had walked around the neighborhood in the morning, in the middle of the night, seeking a possible source. I had asked all of my neighbors in the condo community, my building, adjacent buildings ........ no one heard it but me.

I eliminated electrical transformers, waste water pumping stations, my own blood pressure/heartbeat. Eliminated my tinnutus, which never ceases and manifests as birds furiously chirping. It cannot be heard outside. It is strictly an indoor phenomenon.

It had stopped for a long time, a year or more, and began again maybe a month or two ago. It is not 'on' 24 hours a day, begins at 10:30 or so PM and may or may not continue through the morning. I could still hear it upon waking this morning, but by the time I got back from my walk at 11AM it had stopped.

I googled about it finally and found I am far from alone, and no, I'm not going crazy, at least not yet.

https://en.wikipedia.org/wiki/The_Hum

A long, maybe too long article on the phenomenon ........

https://newrepublic.com/article/132128/maddening-sound