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there are only a few things that would be cause this to happen.
If the chimney has a cleanout on the outside then make sure it is secure and tight. There may be a vent/spark arrestor located on the top of the chimey and if you have one, it could be loosealso banging around in th wind. And next I would check the damper inside the chimney. If the damper is loose or closed ond wind rushes accross the top of it then it can create a suction that will caused the damper to slap.
Check these things and let me know what you find.
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