I agree that it’s probably from a variety of sources, many industrial, and that the experience of the hum can come about or be amplified by stress and heightened sensitivity. In many cases, it may be from a combination of a very real industrial hum (machines, cars, low frequency radio waves?) and heightened sensitivity.
In my case, I started hearing the/a hum a few years ago when I lived in Norway. It seemed to happen or be stronger most nights and on weekends and holidays. It kept me awake and added to the stress I already was experiencing during that time. As with the people in the BBC story, I didn’t hear it anywhere else.
Fortunately, I was able to track it down. It came from a nearby school, and specifically from their ventilation system. Repeated visits showed me that they turned up the ventilation system after school hours and until the start to school the next day, and that it was on during school holidays. It was a very loud sound from an old and rattly ventilation system.
I contacted the principal, and she was fortunately friendly and receptive and had the janitor turn down the intensity of the ventilation system. Since then, my sleep there was not disturbed by this hum.