I have been staying at a friend’s house out in the country, which means I can see mountains out the window from my bed and stars in the sky at night. It also means I can hear the hum. In LA, although there are moments of silence, it never really lasts more than 10-15 minute intervals between 2:30-4:30am. Even then I can hear the highway, or the wheels of a shopping cart against the pavement or a helicopter.
I know some of the hum is from electrical things, the computer, the refridge, lights, but I like to think of the mountains humming, kind of like some Chinese monks trying to connect with something more powerful.