When I turn on the hot water I hear a thumping or low banging noise which lasts for about a minute and will repeat after a few hours when the hot water is used. This only happens when hot water is first used. I have checked the unit and at this time the unit is not heating water nor has ignition occurred nor is the burner lighted. There is no sound when the hot water is turned off. (“Rapid closing of faucets or solenoid valves in automatic water using appliances can cause a banging noise heard in water pipes”).

I have drained the tank and no sediment appears in the water removed and discarded.
I have opened the release valve and it works normally releasing clean water in the drain.
It is not a rattling noise, which I understand is normal.
I have checked the flue baffle & hanger, and it is not generating any noise and is in good condition. We purchased the unit on 3/23/03 so it is less than 6 years old.