Not everyone can hear it (like my wife) and if you live on the water, it's pretty obvious it's the boats. It's when you don't live near water and you hear it at 2am, and go "wow, that's really interesting," and then go to sleep.

I literally haven't thought much of it until reading this article, other than passing observations. For example, I've noticed it's not audible in every house, and usually not audible on the ground floor when it is audible on other floors.