My bathroom ceiling vent does that, too. I find it is the wind that causes it; and it gets worse if I have a window open to make a draft.

Hmmm.. so strange. I've had washers & dryers all my life and never noticed this before.

Sounds like the flapper on the outside is making the noise. Sometimes it happens when the vent is installed at slight angle because of the siding or anything else and that could keep the flapper partially open at all times, therefore even a slight air movement will cause the flapper to bang.

Other times you need to change the exterior vent, get the one with a with a self sealing flapper.

I hope it helps.

Good luck

I would try the self sealing flapper! Is that something I could just pick up at Home Depot?

I'm not sure what HD sells, they probably have the basic dryer vents... Try amazon. com they have something called Deflect-O Ultra Seal Dryer Vent 4 " or try this website http://www.lambro.net/

But before you do, check the vent on the outside and make sure there is no lint build up which also can prevent vent flapper to close tightly and that can cause the problem also.
P.S if your vent location is high up, and you cannot check, call someone with a ladder to make sure there is no installation problems.

Good luck