After two winter seasons of banging pipes, our steam heating system is (knock on wood) basically noise-free now. After ruling out (with the help of Master Plumber) the possibility that the problem was related to water in the pipes, we isolated the banging to an area at the back of the house in the floor between the cellar and garden apartment. After opening up the ceiling from below (which wasn’t too hard to do given the age and condition of the boiler room ceiling), we discovered that our contractor had installed a pair of two-by-four cross supports for the new wood floor on either side of the pipe. As a result, whenever the pipe expanded with each heating cycle it would push against the wood, causing the banging. By putting an extension on our power drill, we were just able to reach one of the two-by-fours and hack away enough of it to put a stop to the noise. It ain’t pretty but it does seem to have worked.