I found roof flashing tape works really well to reduce road noise, its a silver backed roll of bitumen, costs about 10 or 15 quid a roll from b&q/homebase etc and that was enough to do all my doors and have loads left over. The change was very noticeable (I lined each of the plastic sheets inside the doors with it) and was gonna go on to do more of the car, but i sold it before I got the chance!