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.
During a cross country bicycle trip my partner and I were camping just outside a tiny town in the middle of Colorado. I had trouble sleeping and kept getting out of the tent to look around due to what sounded like a large flag blowing in the wind, or a rabbit thumping its foot against the ground. Low, faint, slightly irregular. But there was nothing around. Lasted all night and morning. It was driving me nuts!
