There is a simple way to stop extractor outlets (cowl) knocking/banging in the wind. You will need 2 5mmx 1mm rare earth magnets, there is an example from eBay http://www.ebay.co.uk/itm/161598721229?_trksid=p2057872.m2749.l2649&var=460595392905&ssPageName=STRK%3AMEBIDX%3AIT These magnets are...