There is a shutoff damper on top of the ceiling fan unit. It will open when the fan comes on and close when the fan goes off. Sometimes, when it is windy, the damper will open and close due to a negative pressure in the duct and create a noise. Most people don't do anything to stop the noise because it is not a defect. But, you may be able to find a more quiet fan damper, or add some foam to lessen the noise.
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