Ok just tried holding the mirror neck, no difference, however window down makes it worse. I've noticed a vibration / rattle in the driver's side when the window is down and I'm driving but we'll leave that to another post.

Because it is the only door that makes a Ping / higher tone than the expected solid thump when being closed AND if I tap on the outside of the door with it is closed, latched, window up, the door sounds hollow. I can actually see the middle of the skin vibrate briefly. Like hitting an empty oil drum.

There are no signs of body work prior to my ownership as well the history was clear so I am left to assume that the factory must have left something out.?.