Had a neighbor's house doing something very similar many years ago--loud bang, only a few times a night. Both of us spent some time snooping around, and couldn't figure it out (of course, it never did it while we were down there looking for the possible source). One night it happened when we were playing pool in his newly finished basement rec room. The sound I heard then instantly reminded me of how some steel girder bridges I've inspected occasionally give off very similar sounds when just certain girder lines either expand or contract. Sometimes it's even a loud bang, if the girder is long and deep enough, has fit-up problems, or is restrained by diaphragm connections in just a certain way.
The load-bearing wall down the middle of Bob's basement consisted of a single long run of steel I-beam, welded to support plates at the tops of steel pipe columns. At the time, I suggested to Bob that his noise could be coming from that steel beam line, magnified by the floor joists it carried, as it shortened in a certain manner after the furnace was turned down for the night (a long supply-line plenum ran right next to almost the full length of beam). He agreed with that theory, after remembering he rarely heard the noise during the warmer months of the year, when the furnace wasn't used.
