The purpose of a long tube header (longer than eighteen inches from port to collector merge) is to prevent the high pressure gasses in the exhaust from backing up and entering an adjacent cylinder (such as 5-7) as they fire. This high pressure area that builds as the exhaust opens on seven can rush back into cylinder five if you have a long duration (late closing valve) cam and cast iron manifolds or shorty headers. This wasted gas that rushes back into cylinder five dilutes the incoming charge and makes the cylinder burn lean (acts like an EGR valve). That is probably the source of the noise you hear.
Freer flowing exhaust prevents back pressure build up in the headers, but it allows more noise to get out as well. The factory used a two and half inch dual exhaust on all of their high performance cars. Larger diameter pipe was available back when the cars were designed but the engineers at r the time though that a five hundred horse big block running wide open wouldn't be impeded by the smaller pipe and intentionally chose that size. Chrysler used the same size on their 440 and 426 Hemi cars as did Ford with their 428-460 engines. On the street you can not tell any difference in performance. It you want to race you uncap and remove the extra weight of mufflers and tail pipes.