alot of people i know running bigger than a 4" have terrible resonation at highway speed two friends of mine bought trucks one had 5" and another guy had 6" on his when they bought them and they both switched to 4" mbrp becuase it was annoying and they were not making enough ponies to justify needing that much exhaust
yeah i have the same noise problem. as soon as i get above 70 mph it just gets annoying and thats with my 4". My 4" when i get on it just sounds like a gas truck and i want it to sound like a diesel where you can hear the turbo almost like a powerstroke.
I personally don't think you can achieve the Powerstroke sound...that's all in the way it was built....Duramaxs are probably the quietest diesel engine I've heard....and I've heard Dura's with stacks that are fully programmed and decked out with CAI's and all the goodies...and they still don't sound like a cummins or a powerstroke. But then again..I love my quiet truck. But if you are looking for a louder turbo whistle...then get an aftermarket CAI...but that's all they are good for.