I'm rattling and its bugging me.

Yup, this has happened to me on my 1972 Plymouth Valiant 225. I seem to remember cutting a small diameter rubber hose, much like the washer hose, into small grommets that I fit on the hinge portion the flapper door is attached too, to absorb the shake and lessen the rattleing sound. There needs to be a loose tolerance/fit to allow proper movement. This is why the rattle sound is created. The rubber pieces (just a couple small ones) that I fit to it did not hinder it's function and DID lessen the noise. Hope I was clear!!!!