all part of that constant maintenance. most of the time is from the undercoating put in the doors, it dries out and falls in when you slam the doors. if you discover that it keeps doing it, pull the trim pad. then clean it out from the inside. you will be surprised what you find in there. a little spray bomb of rusty metal primer on the new bare spots and in the pinch weld will keep them safe. if you have a late 60 early 70 MM you will find that any one of the doors may have the drain holes plugged with the over lapping door skins. they were big on that in those years. the only thing that got them to rust. a 1/4 in bit on the drill will open them up. same as those plugged with seam sealer