I think the best you could do would be to have a stock exhaust system and muffler on it. When I tow for ten hours or so in a day I do not want any drone or additional noise at all.
Having owned a 2002 Powerstroke for over ten years, I have found the addition of a diesel fuel additive such as PowerService or something similar will quieten the engine a little.
Otherwise, there is not much you can do, IMO. They are very reliable engines but noisy.
The engine noise has gotten to be the sound of freedom to me and my wife, however!