Outside Dryer Vent Covers Flap

How can i prevent the clanging of my extractor fan vent cover ask question asked 6 years,.A solution ive used for both my bath fan vent as well as dryer vent is to glue a nut machine bolt type as a weight near the lower flap edge.However, my particular dryer run is very short - a few feet, and the bath vent run is no more than 10 feet.