This one seems like a weird one to me.

W110 200d here. Around town there is zero exhaust smell in the cabin. If I drive at speeds greater than 55mph (estimate) with the windows down, I get an exhaust/soot smell in the cabin. Roll the windows up and the smell goes away.

Any ideas on this one? Seems like I'm creating some kind of negative pressure in the cabin with the windows down at high speed. Exhaust leak at the manifold or ?? I have no idea on how to diagnose this one.

Any advice is appreciated. Thanks!