I do not close it and it makes no flapping sound. I realized early on that to try to open it at a campsite and then close it for travel would be too much trouble, so I leave it alone. Now, if it ever starts banging during travel, I will addressee the source of the sound. I have thought of getting up there, by borrowing someone's 12 foot ladder, and perhaps gluing felt cushions to the exhaust door corners. Maybe I've just been lucky, but no banging.
Bob