Desire
Do you have a water softener fitted to your water supply if not adding one would greatly reduce this problem. I do not know the water conditions where you are but if you have “hard water” then I would suspect that the sound you are hearing is clumps of calcium or lime banging against the flueway and the interior base of the tank.
If you have “soft water” then by repeat draining and flushing out the cylinder you should eleminate the problem by washing out the sediment, however, this process is not very sucessful if the problem is due to lime/calcium particals separating from within your water supply.
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