Link to listen to mystery sound:
[soundcloud.com]


We are experiencing a banging sound in our home and cannot identify it. The water heater was recently replaced before this sound started happening, and we are suspicious it is somehow related (though it may be innocent). An expansion tank was also added to the cold water line supplying the new water heater (no expansion tank previously). The sounds have been happening for the past 3 months, but otherwise never ever happened in the past 34 years in this house.

There are anywhere from 1 to 4 bangs. They vary in volume and rhythm. It seems to come from the basement, but can be heard and physically felt all the way on the 2nd floor. We have placed a recorder on top of the water heater to capture the sound when it happens.

It does not happen when turning a faucet on/off, flushing toilet, or other things that draw water. We can't MAKE it happen. It usually happens late at night or early morning, strangely most often on Mondays and Saturdays. It happens when everyone is in bed, or when we're all sitting together watching tv or something, and it happens when no one is home (found when reviewing the recording later). Sometimes it happens twice on the same day, sometimes it happens on 2-3 consecutive days, sometimes a week or more elapse with NO banging sound (that we hear, at least).

A hiss seems to start building up before the banging happens (audible on clear recordings when the basement is quiet).

The plumber who installed the hot water heater doesn't know what it is. We can't make it happen on demand, but have played the recordings for him. He says it doesn't sound like water hammer and sounds more mechanical. But the banging sound happens when the A/C, washer, dryer, dehumidifier, humidifier are all off.

It's driving us crazy and is worrying.


3 of the clearest recordings (with the hiss building up before the bangs) can be heard here:
[soundcloud.com]


Does anyone recognize this sound and its irregular behavior? What is it? How do you fix it??

Thank you for any ideas you may have!

Has the water pressure been checked? With a lazy hand gauge? Is there a pressure regulator? Can you turn the water heater off(so it won't heat) at night and see if the noise still occurs? Where does the T&P valve on the water heater drain to? Can you see if it's been venting?

Thanks for the reply, Steve. I'll answer to the best of my ability!

The water pressure has not been checked, but the plumber who installed the water heater said he could come and check it. Maybe we can find a neighbor with a gauge we can use to test it ourselves. Would closing the valve a little at the water meter be of any use, as far as experimenting?

There is a pressure regulator not far from the water meter.

We could turn off the water heater to test whether the sound happens with it off. In fact, we did do that for one night a while back, and there was no banging. But 2 weeks can go by without us hearing the banging sound, so 1 night isn't a big enough sample size. Turning it off every night would be an experiment to try, but could take a long time to determine if it helps... and then still figure out what that means.

The T&P valve is on the side of the water heater, attached to a copper pipe that points straight down at the ground, ending a couple inches above the basement floor, less than a foot from the floor drain. How would we tell if it has been venting?

The original setup was a 40 gallon water heater with no expansion tank, and no awful banging.
The new setup is a 50 gallon water heater with an expansion tank, plus the banging.

The pressure gauge should be a "lazy hand gauge". This type of gauge has a needle that rises up and stays at the maximum pressure that occurred, should the water pressure fluctuate and allows you to see how high it intermittently rises to. Leave the gauge on for a couple days or until you hear the noise. If the T&P was venting, there would be water leaking from the pipe. Shutting down the main valve shouldn't do anything to help.



Edited 1 times.

I don't remember the floor between the venting pipe and the floor drain being wet when this happens, but will make sure to check that in the future. We'll work on getting pressure readings. Thanks for the replies!



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I would check to be sure that the expansion tank pressure is set above the static pressure of the system. Use a bike tire pressure gauge on the schrader valve on the top of the tank with the water off and the pressure in the system relieved by turning a faucet to warm and letting the excess drain out. The precharge on the tank may have been too low, and if the normal water pressure compresses the air bladder, it makes the tank worthless for dealing with thermal expansion and the resulting pressure spike.