Arrived onsite to investigate a noise believed to be coming from the heating system. The customer reported hearing a loud bang in the middle of the night on a few occasions, though it has not occurred recently. We operated all three heating zones and the domestic hot water zone and were unable to reproduce the noise. We did notice air in the heating zones, so we purged the system and removed trapped air. We verified that the automatic water feeder and air eliminator are operating properly. An estimate was emailed to replace a noisy circulator. The customer was advised to contact us if the noise occurs again. Verified operation; the system is working properly at this time.