Loud thumping, rumbling or bubbling noises coming from your water heater may indicate a number of problems. Water heater noises shouldn't be confused with the noises created by loose or improperly insulated pipes. Noises in the water heater should occur when the hot water is first used, after a few minutes of use, or at random times. Pipes are the most likely cause of rumbling when the water is turned off.
Sediment enters the water heater storage tank through incoming water lines. Leaks in the water lines may let soil and silt enter pipes fed by a well, or the finest particles may escape through the filters at a municipal water treatment plant and end up settling at the bottom of your water heater. Water bubbles forming under the layer of sediment in the tank will cause regular thumping or rumbling noises each time the appliance heats up the water. Draining a gallon of water each month from the tank will help remove excess sediment. This task is as simple as attaching a section of hose to the drain valve and running it outside or into a bucket. Draining water from an older unit may lead to build-up in the valve that creates a damaging leak, so this practice works best when started on a new unit and continued for the life of the appliance.
Thick mineral deposits lead to loud rumbling or popping noises when water trapped underneath this layer is heated. Calcium carbonate and magnesium carbonate enter a hot water tank when they are dissolved by running water from deposits in the ground. Water with high concentrations of these minerals is considered hard. Hard water leaves scale deposits that are difficult or impossible to removed inside a water heater storage tank. Annoying noise isn't the only problem caused by mineral build-up, either. This layer of calcium and magnesium can reduce heating efficiency because it prevents heat from being transferred into the water. Installing a water softener on the lines entering the water heater will prevent build-up, but removing thick deposits is difficult and replacing the unit may be the only option.
Cold water entering the water heater storage tank and mixing with the heated water can cause loud rumbling noises. Cold water is constantly entering the tank when heated water is used, but a leak around the input valve can add more cold water than the unit needs and lower the overall temperature in the tank. Rumbling always occurs as hot and cold water meets in the tank, but when it continually happens this noise is louder and much more noticeable. Fixing leaking valves should put an end to this energy-wasting process.