A loud bang

Last Monday, that would be Nov. 14th while I and my dog were gathering up odds and ends on the second floor of my house, we were startled by a loud crash or bang. Actually it sounded pretty much like a pistol shot very close to us. Shadow, my dog, reacted sharply, running in circles like she was either trying to determine where the noise came from or was just undecided where to run to get away from whatever. The sound was definitely inside someplace. I checked the entire house figuring anything making that much noise must have left evidence of some sort. Nothing was found. My wife was not home at the time and my dog was with me. It wasn't weather related as the skies were clear, cloudless, and the Sun shone brightly. Loud bangs and noises are not uncommon in this old structure. Usually three or four times a year, usually with the approach of the Christmas season. Sometimes we find the cause of the disturbance; but quite often we do not. Needless to say the noise was not in the area where I have the spy camera.