It's going to be your door gasket. Listen to the earlier poster.

Sound insulation isn't really going to change it's effectiveness but rubber door seals shrink and allow wind noise. If gasket treatment helps at all, it's going to be a temp fix.

Replace the drivers side first and you should be see a big diff. Didn't notice your car but 300E's are around $60 a door I think.

Good luck

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Ken Fuller
1990 300E