I remember hearing about this off and on over the years. It’s fun to speculate about the source.
It looks like this humming phenomena is different from the sky songs and earth sounds. From the article, it usually only occurs in rural and suburban areas (less general noise to mask it), only 2% of the population in a reported area can hear it, generally those between 55-70, and it’s only heard indoors, usually at night. I’m guessing this one is more of a physical phenomena having to do with some middle-aged people being more sensitive to a certain low frequency of sound. I already have tinnitus, I hope I never start hearing this thing too!