Join Date: Feb 2010
Location: Ontario, Canada
a common issue with cobalts is the bottom of the doors rubbing on the rubber on the sills, check to make sure those aren't rubbing together.
My sisters has an 08 LS sedan and 2 of the doors are rusted away on the bottom because the doors rubbed the rubber on the sills all that time.
my friends 06 coupe has a rusty driver door from rubbing the rubber that lines the door sill or whatever you call that chunk, its the piece of rubber thats commonly replaced with metal door sills that say "Cobalt" or "g5" or whatever.
Anyways..... put a little black paint, or chalk or whatever on the rubber, shut the doors, go about your daily routine and at the end of the day see if the stuff rubbed off on the door... I "suspect" that's your issue