The gasket that I'm referring to is between the mirror and the door itself. In your pic, that gasket appears to be in place. However, if the mirror is not secured well it can allow wind to get by the gasket and cause wind noise. There is also a piece of foam like this on the inside of the door that is meant to help keep noise down..
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Another thought that I had is; if the windshield was ever replaced and the installer did not get a good bead of erethane along the sides or the bottom edge of the windshield this can also cause wind noise. I have this exact problem with one of my '89's.