knocking noise from bathroom exhaust

We've had this problem since our house was built. I've only recently taken to wondering about this, having bigger problems to worry about!
All our bathroom exhasut vents make and odd knocking noise at the slightest breeze outside. (The fan could be on or off, doesnt make a difference). It sounds as if something is swaying around at the top of or inside the vent, causing the sound. What could be causing this? How can it be fixed? It is starting to get really annoying!

vent

There is a flapper inside the fans outlet and your right. the wind makes it bang a bit.

Many times I have glued a very weak small button magnet near the botton edge of the flapper, by all means don't do it with a rare earth magnet.

The power of the fan can easily open the flapper, but the magnet is strong enough to hold it closed to outside wind.

Paul

I simply used double sided 3M tape and attached a plastic washer at the bottom of the flapper. Worked PERFECTLY. Now it opens when it's turned on and makes ZERO flapping noise when it's not. Hope that helps!

Loungeact

thanks fr your replies. Maybe I should try the magnet (from a fridge magnet?), since it's always a bit windy where I live. Can I reach this flapper from the outside of the house or do I take the exhaust apart??

fan

Take the louvered cover off and look inside the fan box for a flapper. Turn the fan on and off. You should see it.........