You will also hear a bang when hot water turns to steam at a restriction. A strong pump can create enough suction to boil water at a lowered pressure. The steam collapses at a cool place to make the banging.
Look for a pump pumping toward the expansion tank and away from zone valves. The drop in pressure can cause cavitation. This is explained in the system design lesson called “Create a positive pressure” at: http://www.hydronic.net