On my '32 Brookville the doors rattled when I first got it. I found that the builder must have screwed up and left out the rubber bumpers that are supposed to be pushed into rectangular holes cut in the body in two spots and when I got those bumpers from Brookville and pushed them in, all the rattling stopped. Of course my body is steel, but I think if it was fiberglass such a rattle could cause cracking. If you don't already have cutouts for such rubber bumpers, I'd look into Dremel'ing a couple on each door sill, finishing up the hole edge per good f'glass practice (not sure what that is but I'm picturing a raw hole not being good to leave as is). If there isn't already a bumper designed for your specific body/door you'll have to be creative and find one that can be made to work and still let the door sit flush. Just for reference, here's what my bumpers look like: