Hi, this is my first post. I have a hot water baseboard system that recently had the air bleeder valve and a zone valve replaced, ever since the zone valve was replaced I get the hammering noise in the pipes but only when the furnace shuts off after it has reached the tempature. I have spent time sitting in front of the boiler to try to see what is banging but it never happens when I'm watching it. it only does it once in a while but it bothers me knowing theres a problem. the system was bled twice by the oil contractor and I check the schrader valve on the air purge and it only hissed out a tiny bit of air once. any ideas?