There was a house I lived in at the edge of town, and late at night on the balcony (midnight, 1 am, etc) when it was very quiet I could hear really quiet booming sounds. It was quiet enough that if a car drove by on the nearest road it would drown out the sound. It was not a steady hum—more like a distant explosion or bass drum. It was frequent but not regular enough to be music. I would guess it was the 20 to 30hz range. I could never quite figure out what it was.