Take it up to 74mph and it will quiet right down. Seriously, you are not going to blow this thing. I can cruise 70-75 all day long. Granted it is not the quietest car made, but I have been in worst. Just drive it. Make sure all the holes and gaps in the firewall to the cabin are sealed for noise factor. Hood pad helps too if you don't have one.

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1979 240D Caledonia Green/cream gone
1982 240D stick Ivory/Palimino
1999 E300D White/grey