How can I prevent the clanging of my extractor fan vent cover?
I think you just need to find a better built vent hood.  The first one I found searching on HD specifically calls out this problem in the description:


  Unlike other versions that tend to rattle in the wind, the back draft damper on the ProMax cap has been designed for noise and wind resistance and its stylish wide mouth appearance helps to optimize the airflow.


Try shaking it and moving the flaps in the store to see if they make a noticeable sound.