Knocking sound when hot water is turned on

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Gamboo32

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Hello there,

I have recently started hearing a knocking sound that seems to be originating from the wall/pipes near my water heater the moment I turn on the hot or warm water in any faucet in the house. The sound never occurs when only the cold water is turned on.

If the hot or warm water is turned on just slightly (low or medium flow) there is a continuous knocking until the water is shut off. However, when the hot or warm water is turned on all the way (high flow), there is only a single knock and then the sound stops.

To make things more interesting, there may also be a few times (4-5 times) throughout the day when I'll hear a single (slightly softer) similar knock even when no water is turned on.

Tracing the knocking sound, I have noticed that it seems to be coming from the pipe that runs OUT from the mixing valve on my water heater - and not the hot or cold water pipes running IN to the mixing valve. On a side note: The original mixing valve was faulty and had to be replaced over 6 months ago. However, there had been no issues with noise following replacement until just recently.

I've already tried clearing out the air in the pipes several times but the noise is still there.
From what I've read,symptoms don't seem to indicate a water hammer issue either (no sound when shutting off water.
Its the middle of summer so I'm assuming the pipes are pretty warm to begin with, so I figured thermal expansion is probably not an issue either.
I'm at a loss for what could be causing this noise and figured I would ask for some help and cross my fingers.
Hopefully Ive provided enough information to help diagnose the issue.
Thanks in advance for any advice/help!
 

WorthFlorida

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It might be the heat trap on the hot side of the water heater. Google "what are heat trap nipples".
 
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