the motor I mean. Pulling onto the highway 2200~2500RPM It'll all of a sudden get REALLY quiet (almost sounding like a gas motor). That clacking of a diesel simply goes away. Any ideas as to why? Does this happen to anyone else? Could this be related to my P0404 code?

I'm all stock too.

404 code,will not make it quite, as why its getting quite Dont know,but wish mine would:(

Mine does this as I really get on it, no diesel clatter at all, it quiets right down-just pins you in your seat and launches! When I let off the normal diesel noise returns. This is the opposite of what I'm told pre injection turning off should sound like so I'm not sure why it does this. No codes, trucks running great so I'm guessing all is well.

Mine goes quiet also, especially when the truck is on cruise you can barley hear it.
Going down hill the truck doesnt make a sound either like it shuts itself off.

This being my first diesel I'm assuming this is normal operation with these beast's.

404 code,will not make it quite, as why its getting quite dont know,but wish mine would:( x2

At that RPM, the pilot injection goes off or on or something, and the change in the sound is noticable.

Do a search on Pilot injection, it will answer your questions.