Noisy range hood flapper

Hi,
We have just installed a new Viking range hood and vented it out side. There are two flappers, one on the outside wall of the house and the butterfly-type flapper that sit on top of the hood. This one keeps opening on its own, it seems like there a draft or something. The noise is driving me insane. I tried to weigh the wings down, but only solved the problem partially.
Why is this happening and how can this be fixed? I would appreciate anyone's input on this.
Thanks!
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snipped-for-privacy@optonline.net wrote:

My damper opens when a strong South wind blows over the roof. The vent is on the North slope of the roof, and the Bernouilli effect lowers the pressure above the damper enough cause it to open. I can always tell when the wind gets up by the sound of the damper slapping.
I suspect that the one on the outside wall is probably opening at the same time, you just don't hear it. I don't know that there is much you can do about it.
Bill Gill
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