Sticky butyl sheeting or mats will dampen the 'dwang' from areas like the sides, wheel arches and roof.
These are available from a number of resellers. I've bought from DoDo ..
Dodo Mat DEADN Hex is a self adhesive, foil backed sound deadening vibration damper. Simply cut, peel & stick Dodo Mat DEADN Hex on the interior sheet metal of any vehicle to reduce unwanted vibrations & structural noise. Dodo Mat DEADN Hex has a high grade butyl compound for better sound...
www.soundinsulationstore.co.uk
It's very good and has a thick alluminium covering (top).
I have also bought from eBay - this wasn't quite as thick (or heavy) so I fitted this to the roof and under the seats and it's been very good as well. You don't need to cover the panel fully, on the sides and on the roof a single sheet takes out the noise and 'deadens' the panel very effectively. Fitting a sheet to the door skin also makes the doors quieter too.
"The roof of my van now goes thud, not ting. With high grade construction and highly effective damping, Silent Coat Damping Mat is already the first sound deadening choice of many automotive professionals.
www.ebay.co.uk
On top of this mat, in the back I used DoDo again with their 5mm Dodo DEAD-EZY Insulation which I covered with a rubber load mat. I also bought some Bonnet Liner for the cabin area and under the bonnet.
www.dodomat.com
I have tried different makes and to be honest there isn't much between them, technicaly maybe but in the real world nothing I could detect.
Post some picture
@patrickr90 ...
