I think they use a sound-deadening caulk before the paint process or a material/mat between the surface and skeletal frame. You could probably add some caulk to the inside. I haven't noticed on mine...but I will check...my Saturn had a "twangy" cheap sound when closed!I have a 2013 Sonata GLS. Now I know that my Sonata is not made of the same steel from cars of myyouth, but when I shut my trunk it sounds like tin. However, the hood and all doors sound prettysolid. Has anyone else noticed this and find it annoying??