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I am hoping someone can help suggest a next step on a plumbing noise that I can’t quite isolate. House is on municipal water. Closed system. Sound arises after the hot water tank heats, typically after all other water has been off for hours (for example early morning). Sound is a tapping reminiscent of a popcorn pop. The sound immediately stops after I turn on any faucet in the house. The sound is loudest closest to to ingress into the house (away from the tank itself which is on the other side of the basement), near the meter and PRV. I assume pressure is building and relieved when I turn on a faucet. I replaced the PRV and the town swapped out the meter. My expansion tank is new and seems to be functional. I’ve had the hot water heater inspected and no issue was identified. Coincidentally, I began noticing the noise after the expansion tank was replaced , but again it seems to be functional based on a valve check and touch/feel. I’m stumped. Any suggestions for what to investigate next?

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  • you might be a candidate for water hammer arrestor google it for more information
    – Traveler
    Jan 28, 2023 at 1:25
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    Water hammer is from moving water being stopped suddenly. 'typically after all other water has been off for hours" is the exact opposite of that.
    – Ecnerwal
    Jan 28, 2023 at 1:46

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Coincidentally, I began noticing the noise after the expansion tank was replaced

All the details line up to overpressure from inadequate or non-functional expansion tank. So, noticing it starting when the expansion tank was replaced points squarely at that expansion tank.

A new tank can be non-functional if, for instance, the pre-charge is not set properly (with no pressure on the water side.) Or, it may be too small for your system.

You've replaced a bunch of stuff for a bunch of money. You might want to put on a fairly inexpensive pressure gauge with a hi-reading needle to indicate how high the pressure has gone since it was last reset to see what's actually going on.

The symptoms are, again, exactly those of an expansion tank not doing the job it's supposed to. My understanding from your post is that you changed it, the noise started, you absolved it and began replacing other things. You need to look at it a bit more critically.

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