When I bought my Audi A3 8P second hand it had a full set of Federal tyres. The grip wasn't very good in them but above all the road noise was loud!
I decided to upgrade and bought some Michelin Pilot Sport 3 tyres 225/40 R18, the road noise dropped right off to the point that I could hear the diesel engine where previously it was just a continual load road noise. They new tyres were nice and quiet for the first couple of weeks but have now started making a lot of road noise again. I had the alignment done and still the same, it mainly appears when I go above 45.
I wondered if it was due to tyre pressure? What do others with the 225/40 R18 have the pressure at? I can't remember exactly what mine are but I set it at the "2 people & 1 piece of luggage" pressure that is listed on the inside of the petrol flap. I don't have any other issues like vibration just the noise.