Default gave you good advice. I will add one thing.
You may get lucky and you can fix it in a couple of minutes with no
cost. Turn the water heater off. Shut off the water inlet valve. Put a
drain hose on the drain valve and crack the valve. Go around and crack all
the hot water valves that cause that problem. You may want to open a few
others as well. Give it a few minutes.
Shut off the drain valve, turn the inlet valve again, gently until you
get the air out of all the pipes, Shut the valves off and open the water
inlet valve and turn the water heater back on.
You will have cleaned out your water heater and the water hammer may be
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