I just picked up a 2004 lly cc dually with 18k on it no mods and has never had any. At full accceleration the sound of the engine changes it goes from what I would consider a normal sound to what I can only describe as some sort of knock, almost like the sound a gasser makes when you float the valves. My dad has a 2003 lb7 I am not for sure but I cannot recall his truck making this sound at WOT. What do you guys think is this normal. It seems to run fine other than that. Thanks

It's the sound the engine would make all the time if it had no pilot injection. The pilot injection shuts down when you romp on it hard.

Yep, sounds like your pilot injection. It injects a small amount of fuel just prior to the major injection. Makes the diesel much quieter. Cuts out at about 2600 rpm to make more power. Engine gets much noisier than. Put it in 3rd and drive around. When you get up to around 2600 see if it is the same noise.

hey guys I tried to describe this on the ask the tech forum, but got a response Ididnt really understand. thanks for clearing it up. Now that I know what it is, is it normal?

Yep...its normal....this kinda scared me at first because its much more noticeable on the dmax as compared to my old 03 dodge.