Something is binding the glass or is loose causing the glass to get banged around.
There is a bolt which holds the bottom of the wing vent window divider/track to the door. This bolt is prone to rust. Since it is prone to rust, attempts to remove it often causes twisting of the track. This could cause a stress point for your glass.
You will have difficulty seeing this without removing the wing vent assembly.
If this bolt is missing or loose, your glass will get banged around.
Carl
