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I can hear continuous knocking/tapping sound from inside the wall, when not using water.

It is a townhouse. I can hear the sound on upper floor and lower floor.

Water heater is gas powered.

Tried to debug for a while: when I closed the cold water valve for water heater, the sound is still there. But as soon as I open any hot water tap to let the hot water drain out, the sound would be gone.

I kept the hot water tap open (no water out now), and opened the cold water valve for water heater, waited for hot water coming out, and then closed the hot water tap. The continuous tapping sound comes again and does not stop most of the time. Sometimes the sound could be gone for a while.

The water heater is 5 years old.

After I turned off the gas, the sound is still there; I then waited for 1+ hour to wait the hot water to cool down, but the sound did not go away, still as strong as before.

What is the reason of this sound? Any suggestion to deal with it?

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    Does the tapping go away if you turn off the power to the HW tank?
    – RMDman
    Commented Nov 21, 2022 at 1:27
  • When I turned off the gas, the sound is still there; I then waited for 1+ hour to wait the water to cool down, but the sound did not go away, still as strong as before. @RMDman
    – WindChaser
    Commented Dec 4, 2022 at 18:53
  • I t6hink even waiting an hour , leaves the water to be hot enough that it expands the piping. This is an expansion and contraction issue.
    – RMDman
    Commented Dec 4, 2022 at 19:00
  • When it's a cold raining day, the situation becomes worse.
    – WindChaser
    Commented Dec 5, 2022 at 22:12

2 Answers 2

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Do you have a power-vented water heater? Does the sound only occur when your water heater is actively heating?

I have the same issue. The knocking/tapping sound is coming from my PVC exhaust vent. I presume it's from the expansion/contraction of the PVC due to the temp change, and the slight movement of the PVC attached to my home's framing.

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  • 99% of the time, this is the problem. Both copper and PVC expand enough along the length of the pipe that it's critical they not be tight when passing through lumber.
    – isherwood
    Commented Nov 21, 2022 at 17:49
  • It does not have a power-vent.
    – WindChaser
    Commented Nov 26, 2022 at 4:58
  • When I turned off the gas, the sound is still there; I then waited for 1+ hour to wait the water to cool down, but the sound did not go away, still as strong as before.
    – WindChaser
    Commented Dec 4, 2022 at 18:54
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If it's hot water, another possible cause is thermal expansion of the pipes causing them to rub against the holes in joists; friction causes the motion to happen in pulses. (Have that in my own place.)

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