Speedy Jim wrote:
If it's a gas-fired heater and the hammer only occurs when the
flame is on, then it is caused by heavy sediment buildup in the
bottom of the tank. The water boils at the bottom and small
bubbles "explode" into steam.

You will probably get the "Oh, that's normal..."
and, in a sense it is.

There might be other causes for the noise, but that's one.

Jim

I'd assume that would only happen when the heater itself was running. I
get this problem after running the hot water for about 30 seconds.
Would that loss of water engage the heater again?