I have a dripping hot water tap and was going to change the washers. I turned off the water mains and then turned on the hot water tap. I could hear a very loud thumping coming from the water heater. I turned the tap off and it stopped. Sufice to say, I didn’t do anything else after that except turn the water mains back on. The hot water service is a Rheem, gas and is about 15 years old. Any help is much appreciated.