Originally Posted by redline
Turn the "flapper" in the opposite direction so as the wind will not activate it.
Must be missing something here. The "flapper" everyone keeps referring to is nornally located directly between the hood and the first piece of connected duct work. It cannot be reversed because then it will not open when the fan is turned on, and in most installations it is not accessable anyway without disconnecting the hood.
Unfortunately most of them are extremely light, flimsy pieces of aluminum and wind blowing across the end of the duct work often causes them to chatter, especially in short straight runs of duct.. They are intended to keep wind from blowing down the pipe into the house, but mostly they are a nuisance.