Why do our baseboard heating pipes make a loud banging noise after a new furnace was installed?
It turns out the problem was that the zone valves were installed backwards.  They closed in the opposite direction of the flow of water.  Since the zone valves were fighting the flow of water, it was causing a great deal of vibration and banging.  

I had the heating company come back and reverse the zone valves so they operate in the correct direction.  The banging is completely gone.