It sounds like someone is playing the drums inside our hot water tank! The tank is 10-14 years old--except for the noise no problems. After we use hot water (i.e. shower, laundry, dishwasher) the hot water tank is thumping for about 1/2 an hour. It's loud enough we can hear it upstairs. A plumber told us it is due to the cold water hitting the hot water in the tank but we are wondering if there is another reason and if our only choice is to replace the tank. We are noticing more noise in the colder weather we've had recently in Prince George.
Your plumber may be wrong! From many years of experience I know the probable origin of your thumping. Hard water deposits on the bottom of your tank over top of the boiler cap have built up a helmet-like cap. When the burner is heating the water, the helmet of mineral build up moves up and down with the heated water making a drum like noise.