Your water heater can make noise in a variety of ways ranging from hissing, popping, tapping, banging, and even jackhammer sounds. Some are indicators of problems while others are just age and improper maintenance that really are not much of a concern but will go away when the water heater is eventually replaced.
A hissing or, popping sound when the water heater is reheating is one of the more common sounds a water heater makes. When water is heated the water molecules expand taking up more room leaving less room for other things in the solution. The hardness minerals dissolved in the water such as lime feel this molecular expansion and precipitate out of the solution falling to the bottom of the water heater. The minerals build up on the bottom of the water heater and if you do not periodically flush the water heater will eventually coat the bottom of the water heater and may even cover the lower element in an electric water heater. This thick layer of minerals acts as insulation and keeps the water from having direct contact with the element of bottom of the tank. Water entrapped in the blanket of minerals being heated by the element on an electric water heater or, by the bottom of the tank on a gas water heater actually makes a series of small steam explosions as it is heated making hissing and popping sounds. If you have waited this long to flush your water heater the build up is probably quite hard and will not break up with just flushing. With gas water heaters the efficiency of the units will suffer, as the heat does not pass through the blanket of minerals as efficiently. With electric water heaters the build up covering the lower element will also affect heat transfer and many times will cause the element to overheat and burn out. With electric water heaters it is possible when changing a lower element to reach in through the hole for the lower element and break up the minerals so they may be flushed out.
A tapping noise or, buzzing noise when water is flowing can be caused by heat traps installed on the inlet and outlet of your water heater. Heat traps are basically small automatic valves that seal the inlet and outlet of the water heater when water is not flowing keeping the heat inside the insulated water heater instead of allowing the heat to escape into the pipes where it would be lost. Some manufacturers use heat traps that use a mechanism like a marble to seal the heat in. When water is flowing the “marble” moves in the flow making a tapping noise. Other manufacturers use heat traps, which have a flexible flapper type valve. This flexible flapper may vibrate at certain flow levels causing a buzzing sound when water is drawn. These sounds are generally harmless and will seem loudest at the inlet and outlet although the sound may transmit some distance away.
A banging noise when the water is shut off is usually water hammer. Water hammer is caused by the momentum of the water moving in the pipe coming to a sudden stop. The noise will often be a series of bangs louder at first and tapering off then stopping. Pipes that are not properly secured moving from the momentum can be a cause as well as excessive velocities of the flowing water in the pipes. Undersized pipes, and or, excessive pressure can cause the higher velocities. The pressure of your water supply should not exceed 80-psi and if it does a pressure-reducing valve should be installed in most cases with the pressure-reducing valve a thermal expansion tank should also be installed. Excessive velocities in a pipe may cause erosion of the pipe material resulting in leaks. Water hammer is a problem that may cause damage to components of your water supply system and should be addressed.
Jackhammer sounds are almost always accompanied by pulsations in the flow of water. This is usually caused by something loose in the stream of water, which in a sense is turning the water on and off rapidly making a constant series of water hammer type noises. These noises will often vary with the flow volume and may often disappear or, appear only within a certain flow range. Many times the loose debris may be a rubber washer on a valve that has come loose often from recent use. This is something that should be addressed as often it indicates a valve that will no longer function.
In any case while some noises may be harmless and not require any attention some may be serious and require repairs. For your safety it may be best to have a plumber check the noise out for you and determine its cause and if repairs are needed. Some noises particularly those relating to a gas burner operating on a water heater may be very serious and it is better to be safe than sorry.