Boiler Noise

I have gas+hot water heating. 2 zones. Replaced circulator in Jan. Everything was fine until late March. Now have banging noise, on start-up, that sounds like expansion tank noise but is coming from the burner area within the boiler. Expansion tank is hot at bottom, cool at top. Problem is intermittent and most noisy when both zones are on together. What would cause this and what to do? Thank you.