I have herd it myself at yes its at night or early morning never could figure out what it was. Perhaps the drown of tires on the highway 15+km away or the train siting on the tracks with its engines running... I could never hear it outside and it was most load close to the windows as I would guess they were vibrating. I looked into this phenomenon and have read it is not sound we are hearing but rather low frequency electrical or harmonic wave possibly from earth itself. It is not a sound we hear but rather vibration we are perceiving as sound