In our 90s house, there is a consistent knocking sound coming from the forced air ducts (on the second floor). The knocking sound happens every 10-30 seconds when the furnace is on, and for a few hours after the furnace turns off. Short of ripping up the floor or tearing apart the lower floor ceiling to access this duct, I'm just looking for any other suggestions that I could try myself before calling in the pricy HVAC specialists.
I doubt that the sound is from air duct. Since you hear the sound even few hours after furnace stops. Before you do anything to duct, its better to check the sound is not from kitchen or bathroom exhaust termination.
In order for duct to vibrate or bang, it would need static pressure which is normally low at second floor. Sound could also be caused if duct gets hot and wood around it expand/contract. In that case the sound would be from pipe support or boot nails.
Sound could also be from loose damper which vibrates by air flow. Dampers are normally located at supply air outlet right before the boots. Damper also located at basement main duct, but sound from there will got to whole house not just second floor.
There are other sounds that could be from furnace or return duct but it will go to whole house not just second floor. I hope you can pinpoint the sound before damaging floor or ceiling.
All the best!
Thank you for the prompt replies, Dave and Fardeen. We are still trying to pinpoint the source. Do you know of any tools to help detect the noise source? Would a decibel meter help? We are considering inserting a camera in the vent as the ducts cool down.
Our issue is similar to the one in this video:
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