I have an 04 6.0, and deleted my cat and muffler about 20 minutes after purchase. My truck has a nice sound to it if I must say so myself. I was getting things together to do my EGR delete and had a questionable part on my rig. From looking around I think I have narrowed it down to a resignator? From everything I find, they are only on the 03's? right or wrong? I have no clue. I love my turbo whistle sound, and after the years of having the same set up, the hunger for more has hit me. Would removing the resignator (if that is what it really is) A.) make the turbo whistle more? B.) have any negative effects? I have had my fair share of 7.3's, and the 6.0 is a whole new adventure.