Water heater was instaled just weeks ago. The noise was there from beginning of install. the best way to describe the sound would be a mild to hard tap with my knuckles on the outside of the tank. It will also make the noise when the water heater is not fired. Example, Turning on cold water from a tap. Thanks for any help. I called AO Smith and they wouldnt tell me much, But they said it sounded like water hammer. The house is 65 years old and the water pressure is noy very good so I dought its water hammer.