Sound deadening! Don't use that butyl roofing crap.
Loads of really cheap sound deadening on eBay.... I got a large roll of Vibe deadening for 20 quid so no need to "make do" with something that isn't fit for purpose. The roll would easily do one entire door with a bit over.
This does 3 square metres and is top quality stuff at 6mm but you can get 2/3mm for way less the DODO stuff is really good.......
http://www.ebay.co.uk/itm/6m-Roll-Car-V ... Sw6DtYWnbL