If your water heater makes loud, rumbling sounds, you most likely have sediment buildup at the bottom of your tank. The sound is actually the water inside the sediment starting to boil. The sediment buildup causes heat to be sent up the flue instead of being transferred to the water. This decreases the efficiency of your water heater.
You may be able to remedy the problem by draining some of the water from the bottom of your water heater tank. You do this by attaching a garden hose to the valve located at the bottom of the tank and allowing the water to drain for approximately five minutes.
The water coming from the unit is extremely hot and dangerous. The water should only be drained into a bathtub, laundry tub, or down a floor drain. Do not drain the water into your toilet bowl because it will crack. You also should not drain the water onto your lawn to avoid killing your grass.
If you have an old water heater, you may benefit from replacing it. Newer model water heaters are more energy-efficient, and many new models come with features to prevent sediment buildup.
Gas water heaters sometimes make a sound similar to water hitting a hot skillet. This is a result of the natural combustion process and is normal during times of peak demand, if the tank is in a cold environment, or when first starting a new water heater that is filled with cold water.