Ummmmm....Borla is the quietest I know of, but they tend to get pretty loud with out cats. You could always try welding in a Magnaflow resonator before each of your current mufflers. They are pretty much "straight through" performance and are pretty good at taking the edge off loud exhaust systems. But I have never seen a Mustang with more than two mufflers, so I don't know what kind of adverse effects this would have on performance.

 

i ran the hooker max flow's they sounded good and got rid of the drone, but instead of a drone i had a loud boomy sound(they were dumped too though)... it was better than the drone, but i wouldnt call it all that quiet....... definatly quieter than my flows.....


i think most stangs the h pipe itslelf makes that resonate frequency which is hard to quiet down........ you can get rid of the drone but as far as having an actual quiet exhaust..... i dunno ......

quietst mufflers i've heard with no cats was the dynomax super turbos........