Some areas to check for noise.
- Turbo up pipes - These are notorious for leaking and can make quite a racket as it is right next to the cab
- Exhaust back pressure valve - If it is sticking it is noisy and bleeding boost. You will get a loud "whoosing" sound if stuck
- Exhaust and how it is mounted - It should have rubber mounts to isolate the vibration. If they are gone it can make the exhaust vibrate the frame and the harmonic can be maddening
Ford made a firewall sound deadening kit for the Excursions that can be added to the firewall of the Superduties. There is also a lot of sound leakage around the steering shaft where it goes through the firewall. I have seen a couple of mods with a foam coozie used to seal it up. The other big helper is good rubber window and door seals and sound deadening product in the door panels. Biodiesel also makes a huge difference in how loud my truck is. A HPX crossover pipe helps too and so does a in tank fuel pickup mod as the fuel system sucks air which makes the injectors really noisy.
There are lots of things to do but no silver bullet. The 7.3 is loud.