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materials tend to expand when they get hot, and that is true for sewer pipe material as well. Whether it's cast iron or ABS, the pipes will try to expand when they get hot and when they try to get longer they'll slide across wood or other materials. The "knocking" noise is the pipe fitting tightly against the wood and when the pipe has expanded enough it moves suddently and for a short distance, producing a "pop" type of sound. If you can expose the pipe you can cut out the wood where it is tight and possibly put in other material so that the pipe doesn't rub so hard.

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18y ago
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13y ago

That is usually called "hammering" and is caused by air pockets; it can be reduced by installing a device to arrest the air pocket. A DIY person can do this, check for the devices in a local home improvement supply company, they do not cost much.

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Q: What causes knocking in drain pipes when hot water drains?
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