That map looks just like a population map of the USA and UK, plus assorted English speakers from western Europe. Combined with the fact that the sound seems to grow stronger indoors, I think the source is inside their own heads. Not in the sense that they're crazy, though. Maybe as people get older and their inner ear changes slightly, the signals that are being sent when everything is silent also change, but the brain doesn't. So they interpret the "silent" signal as "the hum".
My first test would be stuffing their ears and putting them in a perfectly isolated acoustic chamber to see if they still hear it. I'm pretty sure they would.
My first test would be stuffing their ears and putting them in a perfectly isolated acoustic chamber to see if they still hear it. I'm pretty sure they would.