If you are in an area that has “hard water” then the problem will be caused by lime or calcium buildup within the cylinder. When the gas heater burner lights up it has to heat through the metal of the cylinder base, then through any sedement that may be there and last of all through the build up of lime or calcium to get to the water. The result of all of this is that flakes or chunks of lime break away and thump against the cylinder walls or base and create a rowdy and annoying noise.
Sometimes if the lime deposits are small they can be removed by draining down the cylinder and flushing it out – otherwise there is no real cure apart from a new cylinder and to prevent a repeat of the problem install a water softener to treat and remove the hard deposits before they get into the cylinder.
Hope this explains the situation to your satisfaction.
Selgas Services Ltd
Craftsman Gasfitters, Plumbers, Electrical Service Technicians