Well for the 5.9 up until 2003, one word Mechanical Injection Pump and Mechanical injectors! This is also known as the Big Bang theory!
The noise is about 50/50 between the two, once warm they quiet down a little.
Once common rail and electronic injectors came along, things got quiet in a hurry. I test drove a 2010 Ram/Cummins and if I hadn't popped the hood first I would have sworn it was a gasser!
I can replace my entire fuel system for about $2,100, that is Injection pump, six new injectors and new lift pump (low pressure fuel pump).
PS: (on edit) I bought mine used, but got the build sheet from Chrysler, mine has the extra sound deading package. Still a little noisy in the cab.