I've always noticed at motorway speeds that there was more a little more road noise coming from the rear on the near side compared to the off side. After 2 years of ownership I decided to investigate by removing the rear seat squab to see if there was anything obvious that could be rectified. What I found was a circular metal access hatch which hadn't been very well fitted allowing an air gap. This picture I found isn't my car or in fact even a W204 but the plate is the same. You can see it isn't exactly flat and that there is a cable entry from the very bottom of the picture which means it needs some care and attention in fitting if it's to make a good air tight seal. Well they hadn't used enough care and attention so I removed and refitted it ensuring it sealed properly. Result ! The higher level of road noise on the near side has gone and the car has a new feeling of refinement. Only wish I'd done it 2 years ago.