You could take it to a car detailer (or similar bussiness), who can apply a spray on product, similar to body schultz. I know of one here in wollongong that does it for all new BMW's and Falcons. Apparently it reduces the outside noise heaps. I think that it is appliedd to the wheel arches and the under body. Problem is, you get tar like black crap all over the underside of your car. However if they are careful during the application there shouldn't be much over spray on parts that may require mechanical attention in the future. I've been considering doing it to my car for a while now, if I get it done I'll post back and let you know how it goes