An account in USA, Rockford

Don't notice it during the day when there is sufficient ambient noise to mask it. Don't notice it when I got to bed because I set the TV on automatic shut-off so I won't have to hear it. But if I wake up in the night with no TV on, there it is. It irritates me but I've spent years training myself to ignore it, so usually I can get back to sleep. If I can't, I sit up and turn the TV back on to mask it. It's a very low-pitched sound that my wife cannot hear and to me sounds like it could be mechanical. Could be. I don't know. I have sort of assumed there must be an underground sewer line pump nearby, that's my go-to guess. But I have heard this is multiple places around the U.S. so... I had no idea until seeing an article today that many other people hear this sound around the world. That kind of makes me feel silly for thinking it was sewer pumps. In my defense, I did NOT hear it whenever I lived high enough, which has occurred twice, once on the third floor of a townhouse in Atlanta and once about 2 1/2 stories high in Reno. So this reinforced my notion that I was hearing sewer pumps. Now that I know other people hear it, I'm tempted to ask the local sewer authority for the location of their pumping stations, just to check if my theory has any likelihood at all.