The slamming noise you are hearing from the passenger side footwell area is actually coming from behind your glovebox. It is the outside air flap to the heat/AC system. It has two vacuum pods on it, a short throw and a long throw pod. One of those pods has a leak in the diaphragm and when you shut the car off it is supposed to slowly close the outside air flap. When it leaks vacuum too quickly, the door slams shut. I replaced both pods on my 86 300sdl this summer and it is very quiet now. I have not changed the ones on my 85 380se and it is very loud!

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John
1986 300sdl
1985 380se