Hey guys I have a '02 5.9RT. I have a Flowmaster American Thunder Series installed but I'm not quite satisfied with the sound. It is loud I guess but it's such a high pitched noise. I wouldn't expect that from a truck with a big engine. I would like some more rumble from it because the motor is so quiet. I was thinking about a cold air intake system ... possibly K&N. What can I do? Thanks in advance.

A cold air intake will not make it louder. Cut off the resonator (not sure if you have one) or switch to another series or manufacturer,

I personally like the sound of Magnaflow myself.
You looked at the 50 Series HD - Three Chamber Mufflers ?? they sound pretty good and have that deep sound to them
Headers, if not will make a big change in sound.

Nope. No double setup on the Gen1. No resonator.

IndyD

I originally had the 70 series Big Block II but that was so quiet so I changed to a 40.

You could always go glass pack.

lol ya that would do it.

Are there any mods to the motor that will give the D a more aggressive sound. I'm looking for something other than an aftermarket muffler.

You can do a bore and stroke or port the heads, not that this will make it louder or sound more aggressive. It will make the engine have a distinctly different sound. The exhaust system is "muffling" the sound, hence the name muffler or silencer in Britian,
FF

Headers can change the tune and piping but as fascict said with the upgrades as well.