I had the noise issue in my VW Komb, I have purchased soundproof matting from Amazon, expensive but excellent. I removed the rear wheel arch liners and soundproofed the rear arches with Dodo mat before replacing the liner. The road noise has been reduced and is alot nicer to travel in.
Dodo Mat DEADN Hex sound deadening mat, 20 Sheets, 20 sq.ft (1.8sq.m), car & van sound proofing, vibration damping