Customer was reporting that there is the occasional pop in the area of the water heater and hydronic air handler for heating. Found failed and flooded expansion tank. While testing faucet a notable thumping could be heard at top of water heater when faucet was opened and closed rapidly. This is caused by water hammer. Tested heating by cycling five times without any problems. Drained water heater slightly and replaced 2 gallon expansion tank. Bled air from system and checked for water hammer. Could not detect water hammer at tank after replacement. Customer will monitor. If sound persists we can run full diagnostic on heating system. Gave option for new water heater. Water heater is 8 years old.