Where I hear the hum there is no electricity. We are miles
from the shore.

I used to think it might be a frieighter engine...but those rumble.

This sound I hear is a hmmmmmmm--and it is somewhat melodious. One note like from an organ key left down. It is not high, not very low either. And sometimes it zooms up in pitch to another note a bit higher.
I've tried to "control" the pitch change, but I can't hold it for more than a short time say 30 sec. So I came to the conclusion that it is in my head. I thought maybe blood pressure related.
BP comes down at night.

I started the nitrate in October last year when we got back. No hum at home since then. But I never had it here much, anyway.

Sounds carry over the water quite a distance...and my husband can hear the freighters coming long before I do. So I don't think they are the source. Each has a different type of engine.

I really don't think about it much, past the first time I heard it.
Kevin and I were alone that summer, so it must have been more than 5 yrs come to think. He didn't hear anything, and he has perfect pitch. It was disturbing that first season I'll admit. I can screen out sensory things with autohypnosis, so I just learned to do that. I am not freaked by it, like the woman in the link.

I think it is interesting that more people are reporting it. I thought it was a New Mexico thing.