The rear door is separating from the rest of the door panel. Looks like the factory used some sort of foam adhesive that is now completely separated. Makes for a nice rattle.

Anyone repair this before?

 

stuff some more foam down there. Glue it in place.

 

What type of glue/foam will hold up?

 

that would work. there is also: bodyshop supply store or good part store and ask for panel adhesive. urethane for windshields works also but the adhesive is better.

 

Something springy. Rubber foam such as neoprene. ( I think GM uses an expanding spray-on product but it's probably expensive)
Or polyethylene foam that electronics sometimes come packaged in.
Then you'll need a polymer safe adhesive. Window sealant would probably work.

 

Typically the material is foam backed and you can stick it down with rubber cement, the industrial kind you get in a hardware store. I did interior panels for my cargo van that way and they've held up fine.

 

What I see in pic, is some aftermarket dynomat