Your door has a pad of sound deadening material stuck on the metal. If your car is before 1999 it will have a plastic sheet protecting against water getting on the back of the door panel and into the interior. After 1999 the sheet was replaced by a rigid foam liner, which also increased the sound deadening of the tinny door. When I was doing maintenance inside the door of my 1997, I replaced the plastic with the foam, and it quieted the closing some.Is your doors makes a hollow sound when you shut them ? Mine are pretty bad at that... Sounds like a van door !
There's a product made for that:... didn't like the outer skin of the door vibrating by the speakers in the door. So we glued "roofing" on the inside of the outer skin of the door (I don't know if it also called roofing in English, it's basically what is put on flat roofs)... meant the door was a little heavier, but the sound when you closed it.....a phenomenal, deep, solid sound
It's called flash band in the uk, see my first postThere's a product made for that:
http://www.raamaudio.com/vehicle-fit-guide/?mat=8&sheet=1
Bituminous mastic on a thin sheet of lead.