The customer stated a loud banging noise. Found the unit off, turned it on and heard the flame and gas valve turning on and off quickly. Found water in the flue pipe causing the pressure switch to open and close a bunch of times and causing this “banging noiseâ€. Found the trap clogged and one rubber 1/2 inch 90 a little back pitched. Cleared the drain and cut the rubber 90 down to fix the pitch of the drain. Tested the system operation after.