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.

>it's pretty obvious it's the boats

What exactly about "the boats" would it be? The motor? The propeller?

Doesn't sound like a thing...

It definitely is a thing; see my reply above. It's the engines. I was able to experimentally connect /one/ source of hum to tug boats specifically. Interestingly, though, the huge container ships that passed by did not make an audible noise.

One unfortunate effect of the name "the hum" is it kind of implies a single source. Realistically, it's probably a whole collection of sources, including some real physical phenomena, and some tinnitus-like audio hallucinations.

Not sure what it is specifically. When living on a sailboat, under the waterline, you can hear it quite distinctly.