For about 3 months now I have gotten a loud thump / bang noise from my water heater. It's a single noise that usually happens once a day after taking showers, sometimes randomly other times. No more than 3 times a day. The water heater is about 1yr old. It's a heat pump style 80 gallon water heater. I turned off the heat pump to use electric elements only and problem still exists. The first thing I tried was to drain out the water heater. After refill, the problem still existed. Second, I turned down the water temp. I had it set at 138 and bumped it down to 128. Still no difference in removing the noise. I continued to look into problem deeper and I discovered that my expansion tank had been failed and full of water. It was apparently undersized for the 80gal heater anyways. Figuring that was the problem. I bought a larger expansion tank and charged prior to install at the pressure of my water. I get 60psi as tested at water heater and around 50-55 at my outside spigots. I set the expansion tank charge at 50psi. I'm still getting the thump noise. It seems to be triggered most commonly following showers or high hot water usage. It would seem tied to heating/expansion of hot water. I don't know if air in the line would cause a single random thump like this or not. There is no evidence of a problem while using the showers or faucets. They don't hiccup and function just as they always have. My pipes are all copper plumbing and the house is about 7yrs old. Any thoughts or help would be appreciated.