My hot water heater recently started making all sorts of noises, akin to water hammer, but only when the burner is on. I had a plumber come out to check it and he installed a surge device on the cold water inlet line. This cured most of the problem but there is still noise when the burner is on. I have bled the unit to get rid of any sediment but nothing much came out. The fact that the noise occurs "ONLY" when the burner is on seemed to puzzle this plumber sent by a reputable company.
Any ideas of what else to do except install new unit. This one is four years old.