Don't use Second skin, buy from Don at SDS. (link in my sig) Better bang for the buck, in particular when advice is included. Rattles are different than deadening. Deadening only takes panel resonances out. If you have rattles you have to find them first. Usually some CCF in between the surfaces that are rubbing will solve those issues. And yes, it is very possible deadener will reduce your score...although it might improve it, depends on a lot of factors. Using CCF to kill the rattles won't hurt your score at all though.