Several years ago I was was working late on a plane in my basement shop . I kept hearing this dull thud sound evey few minutes . I kept working until my curiosity peaked and started looking for the noise . I located the sound to be in the far corner of the basement in an area that was used for storage. Inside an old 75 gallon fish tank I found a mosue running back and forth hitting the glass at each end . I thought that was pretty odd but only oart of the sound I had been hearing . I soon found the other part of the sound to be an 6 foot black snake striking at the miuse and hitting the glass. I dont store old fish tanks anymore .