Hydronic heat banging noise

dafl12

I have a hybrid hydronic/forced hot air heating system. The water part has started making banging noises. The noises appear to occur randomly and unpredictably. The noises are very much like the noises that steam pipes and radiators make when the system has developed steam pressure and condensate is getting hung up, not returning to the boiler.

The system pressure is normal--15psi approximately. The water temperature is well below boiling. Under these conditions, what could be causing the banging? Cavitation at the circulator? (It is a cartridge type, and seems to be working properly.) Can cavitation occur elsewhere (besides in the circulator)?

Also, the 30psi relief valve has dribbled a bit. Can pipe banging as I've described cause enough of a pressure spike--15psi--to activate a relief valve?

hot_rod

Does it have a zone valve? They can sometimes bang when they close