Could also be the padding within the door.
After the Coupe was whacked by a deer a couple of years ago within several months I noticed that the door sounded tinny when closed. There were times when it made a nice "dong" sound when closed, like someone smacking a giant gong.
Took it back to the body shop that did the initial repair & had the car back within a couple of hours. The service manager said the the door insulation had fallen off. They just glued it back in position and voila', end of issue.