My house was built in 2000. For the past year or two I have noticed a loud banging coming from the interior wall behind the water heater. Critters have been ruled out. The noise is not correlated with turning water on or off. A contractor looked at it and was baffled but said it is not coming from water heater. I turned the heat off in case it was the air/heat ducts and still hear the noise. What could cause this? It's driving me nuts. It bangs for a few seconds and then stops and does this on and off all day.

Shut your water main off, open a faucet to blow off pressure. Did the sound go away only to return later when the water is back on?

Would this be a gas or electric water heater? Let me know, Tom

ballengerb1: I have not shut off the water main yet but since posting my question I have realized that when I turn the water on the banging stops. When I turn the water off the banging starts within about 2 minutes or so.

speedball1: this is a gas water heater.

Shut your water main off, open a faucet to blow off pressure. Did the sound go away only to return later when the water is back on?

Yes, the sound goes away when the water is turned off.

A few things come to mind. One, you were wrong when you stated:
The noise is not correlated with turning water on or off
Because later you said:
since posting my question I have realized that when I turn the water on the banging stops. When I turn the water off the banging starts within about 2 minutes or so.
So let's start from scratch. Several things come to mind.
You could have water hammer **OR**
If you don't flush a gas hot water heater on a regular maintenance schedule it will develop mineral build up on the floor of the boiler. Water will seep under the mineral and when the fire comes back on it will generate steam. The steam escaping from the mineral makes a loud sound.
To check, turn the thermostat to pilot and let it set for a while. After a few hours if the noise doesn't return you've solved your mystery.
If you don't flush your heater let me show you how.
For long life and fewer troubles you should keep your heater clear of mineral build-up by FLUSHING NOT DRAINING on a regular schedule. Attach a hose to the boiler drain at the bottom of the tank. With the pressure on, open the boiler drain and let it run until the water runs clear. You will see a spurt of red,(rust) followed by white or yellow grains,(lime or calcium carbonate). This shouldn't take more then a few minutes. Do this monthly to keep it clear.

The next thing that it could be is water hammer.
"Water hammer" happens when you abruptly shut off the water. You have at least 40 pounds of pressure pushing the water flow. The weight of the water hitting a 90 degree bend causes the sound that you hear. We put in air chambers to cushion the hammer much like the shocks on your car.
Back in the 60's we were required to to put air chambers on all bath lavatories and kitchen sinks. Over time water and condensate built up in them and they must be recharged. First turn off the house at the main house shut off valve. Then open hot and cold faucets at the farthest bathroom to prevent air lock.(This is important.) You will find under each fixture little brass or chrome valves. These are called angle stops. With a small pail held under the angle stop to catch the run off look under the handle of the stop and you will see a nut with the stem running through it. This is called the bonnet packing nut. Put your wrench on it and back it off counterclockwise. Now open the stop and remove the stem and washer assembly, The stop will began to drain. When it has quit draining reassemble the stop and move on to the next one. Do this on each one of your angle stops. There will be six of them in a two bath home. When you are done, close off the faucets you have opened, turn on the water to the house, and give yourself a pat on the back.
Hope this helps, Tom

This sounds like a water hammer problem. But since you are not running water when its happening, I would suspect a toilet fill valve is turning on and off, due to a slow leak at the flapper valve in the toilet. Shut off the valves under the toilets and let us know if the noise goes away. IF it does, we will take the next step.

Since posting my last question I had a plumber come out and he said that I had high water pressure and the PRV needed replacing. He installed a PRV at the street level to take care of pressure coming into house as well. Also put an expansion tank on. The problem completely fixed the pressure and banging pipes but now I have loud booms coming from bottom of water heater when water is turned on. I did not have this before. What gives? Plumber says he is baffled. I asked if the heater needed to be flushed out and he said no. The water heater is a State Select gas and is 10 years old. Prior to the new PRV and thermal expansion tank I never heard a sound coming from the water heater.

[I asked if the heater needed to be flushed out and he said no.Your plumber was mistaken and gaqve you bad advice. ALL water heaters need to be flushed on a regular maintenance schedule. No exceptions! Let me repeat.

If you don't flush a gas hot water heater on a regular maintenance schedule it will develop mineral build up on the floor of the boiler. Water will seep under the mineral and when the fire comes back on it will generate steam. The steam escaping from the mineral makes a loud sound.
To check, turn the thermostat to pilot and let it set for a while. After a few hours if the noise doesn't return you've solved your mystery.
If you don't flush your heater let me show you how.
For long life and fewer troubles you should keep your heater clear of mineral build-up by FLUSHING NOT DRAINING on a regular schedule. Attach a hose to the boiler drain at the bottom of the tank. With the pressure on, open the boiler drain and let it run until the water runs clear. You will see a spurt of red,(rust) followed by white or yellow grains,(lime or calcium carbonate). This shouldn't take more then a few minutes. Do this monthly to keep it clear.

Good luck and begain flushing your heater. Tom