Flapper box noise?
 

The weirdest thing- when I shut my doors, the flapper box on that side of the car makes a "clunk" noise. I checked the G-pipe I installed in place of the secondary muffler. Not it. Finally I called my wife out to open and close the door while I was under the car. It took me awhile to figure it out, but it for sure is coming from the flapper box. In fact, by chance, she opened and closed the passenger door and it affected that side of the car.

What the heck can cause this? Is it normal? FYI-My heat and air seem to work great. I am mystefied.