Common cause is the lower edge door trim rubbibg against the rubber lip of the plastic door sill guard (also known as the kicking plate).
I used a thin roll of padding, a strip of velcro pad (the non-hooked side), and pasted it to the edge along the entire bottom trim on all 4 doors and it has worked well.
I would suggest you try it on the affected door before doing all four.
Hope this helps.
1983 w126 280SE - daily drive
1983 w123 200 - sold