I've got this problem in my house.

I live in a 41 year old townhouse, and whenever after I turn off hot water all of a sudden a pipe somewhere inside the walls starts to bang.
It's not too long, but it's definitely annoying. I can't tell where exactly it is coming from, which pipe that is.

I cannot hear the sound from my basement, only from certain parts of the house. It continues for about 2-5 minutes after I turn off hot water; after shower, or flushing toilet or just after using a faucet. It seems that the noise starts when the water is going down the pipe.

This problem only occurs in the winter or when it's cold outside, never in the summer.

I've tried draining the pipes in the house, turning off the main hot+cold water supply and opening all the faucets that is; did not help.

I have no idea how to resolve/fix this issue.

I would really appreciate some help identifying the problem or giving some suggestions on how to fix it !

Thank you very much !

Yep, had that problem before. I thought it was an airlock too and drained everything.
In my case it turned out to be a problem with the ballcock on the feeder tank.
It was bouncing up and down, sending a pulse of water down the pipes.
I replaced the float on the ballcock and the problem went away.

Apparently, this problem surfaces when the local pressure changes.
You have a float for the old pressure, not the new one.
Local pressure usually changes if there is new building work in your area, but it can also be seasonal.

Interesting solution by feroluceb but not everybody has a "feed tank".
More the likely your air chambers need recharging.
"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.
You have just recharged your air chambers and saved yourself a hefty service call bill. Hope this helps you out and thank you for rating my reply. TOM

Are you sure it's not a plastic pipe which is rubbing against the woodstud when hot water is running ?

Are you sure it's not a plastic pipe which is rubbing against the woodstud when hot water is running ?

You, Tom

Quote:

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Are you sure it's not a plastic pipe which is rubbing against the woodstud when hot water is running ?

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Let's explore that. It's called expansion and contraction.
The noise is more like a tapping then a thud, so which is it? Back to you, Tom