A cam does not nessacerily make a engine louder at idle. It can allow the engine to run extremely rough and sound louder. Create a vacuum leak and a stock cam will be lumpy also. Its the cats that are muffling your engine. If you have stock mufflers I'd get rid of those and throw on some 40 series flowmaster mufflers, and an x or h-pipe. An x-pipe will give you a more aggressive sound, h-pipe which will just give you an obnoxious drone sound, but loud also.