I got a call yesterday from a past client (a year ago) who is complaining about occasional popping or banging in the walls of the house. He says it does not seem to correspond to heating and cooling as it sometimes occurs at 5 am, before the sun is up. I questioned him about water hammer and drain pipes, he said it occurs when no plumbing is in use and he does not think there is plumbing in the wall. The home is stucco with few or no exterior cracks and no interior drywall cracks, so major expansion and contraction doesn't seem likely. No evidence of any foundation movement. He says it is loud enough to wake him up. Before I go over next week, any ideas other than ghostbusters?