Quote:
Originally Posted by CTYankee
If a change of filter doesn't do the trick it could be an air duct flexing. It's a common source of noise that occurs when the pressure inside the duct lessons when the system shuts off.
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Yes, this is usually what causes this noise. I had a house in Nashua with a forced air system that did this. The thin wall duct systems that don't have adequate bracing do it all the time. They expand (belly out) when under pressure and temperature and suddenly contract (belly in) when the fan stops. It can be worse if the ducts are insulated because the metal gets hotter.
Has this just started or has it be doing it all along?