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When my furnace shuts off, there a knock sound after it shuts off, is this serious ? What is going on

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It will depend on the type of noise.
Most common is the single "Knock" coming from the return air duct "relaxing" after the blower stops.
re: Blower creates a vacuum on the intake side which may slightly collapse the return duct, then it will pop back to its original position when the blower turns off resulting in a popping/knocking sound.
If the offending duct work is exposed and accessible, you may be able to dampen the sound by adding insulation tightly around the duct, or some other means of preventing the duct from collapsing/relaxing during use. Be careful not to reduce or overly restrict air flow through the return duct which may reduce your furnace's performance or shorten blower life.

Posted on Nov 16, 2014

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Posted on Jan 02, 2017

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