I suspect that you have excessive pressure from your house water main, and that there is probably nothing wrong with your water heater. You probably tee’d off your house cold water lines from your WH cold inlet, and when a cold fixture valve shuts you are getting water hammer transmitted back to your WH. You should ahve a pressure limiting valve at the entrance of the supply line to your house. If not, intall one. As regards water hammer, you can install water hammer arrestors at each fixture, or better yet, install a multitube arrestor on the supply line where it enters the house. And, of course, make sure that all of your pipes are securely held to the structural members of your house.
Finally, you could install an expansion tank atop your WH, but this helps only if your noise is due to hot water expansion, which you state you have ruled out.
NtP
