I have a sound, a bang, that occurs a few time a night down cellar. I've had the oil furnace company check things out, no problem. I've had someone near the furnace when the heat kicks in, no noise. I've had someone near the water heater when it kicks in, no noise. As far as I can tell I don't have an animal problem. No smell or other signs. It always happens at night and only happens a couple of times. I don't believe in ghosts so I'm ruling that out. Anybody have any ideas? Thanks a lot.