The license plate frame was the first place I checked, and after various attempts at tightening, loosening, and padding, I realized it wasn't the source of the rattle on my car — I had to dig digger for a proper fix, and while doing so, putting sound insulation in the hatch itself provided further improvement.

 

Glad you found it!! It's funny how one noise on a quiet car can be soooo irritating. But driving an older car where everything squeaks and rattles is a comfort (perhaps because if it's quiet it probably fell off lol) I used to drive a 1967 vw bug

 

I would also check the lever on the seat to be sure they are locking. Hope you can narrow down your options.

 

Thank you SO much! I would have never found the source of my rattle without your post.