MK240d, I live out in the country.... and one thing which can be amazingly loud is the tires themselves hitting the road...
I don't know if tires are rated by noise... but you might check that out and consider , if not different brands, at least different tread patterns.....
If you clean the wheel wells and spray undercoating from one of those aerosol cans it can really help... but follow directions and shake often.. and use old rags bound around the arm and plastic gloves... very sticky and splatters some ... eye protection good thing also....two applications will help build up the thickness.. what counts is how much air you can trap in the mix....
I also have a 1980 240d... is yours manual ?