The working theory right now is that the Hum is the body’s reaction to VLF radio energy. You can find detailed descriptions and further discussion of this theory elsewhere on this blog.
And there is a simple and elegant experiment that will sort out whether there are multiple hums, and whether some of them have acoustic or internal sources. I call it the “Deming Box” experiment, named after the geoscientist David Deming, who was one of the first serious scientists to come forward with a sober and well-researched paper on the worldwide Hum. It comprises three boxes which appear identical on the outside and to anybody inside them. One of the boxes will greatly reduce acoustic signals (i.e. soundproofed), while the second one blocks VLF (EM radiation between 3 kHz and 30 kHz). The third box is a control and blocks neither.
The challenge at this point hasn’t changed: I need expert opinion on the construction of the boxes. Funding is a separate issue.
The experiment, and the solution to the source of the Hum, awaits.