The Hum had manifested itself to me on two occasions on the West Coast.
The first time was when I was staying with a hostess from couchsurfing.com near Monterey, CA. Each morning as I woke and all was quiet, I could hear that faint hum, like a gigantic engine from a distance.
The second "session" was the two days I spent at the USA Hostel in Hollywood. Each morning when everything most all was quiet, that faint electric pulsating hum could be heard prominently.
Spaceship Earth! LOL