My dad's '99 Cummins is pretty loud. My '04 isn't nearly as bad. Inside with the windows up, you can hardly tell it's a diesel. I can even order stuff at the drivethrough without shutting the engine off.
Of interesting note, I heard that when Cummins was redesigning the engine for the common rail high pressure fuel injection setup, they had a focus group come in. The group was composed of "diesel guys", and they said the engine was "too quiet", that you couldn't tell it was a diesel. So, Cummins made it a little louder.
Don't know if it's true or not. Seems odd to me...I'd make the thing silent if I could. But maybe the diesel heads out there want it to sound like a Kenworth.
Bottom line: I really like mine a lot! I used to have a 350 powered Suburban. We were coming up Corridor H in WV last weekend. A pretty steep hill. I'm running about 70-75 and the Dodge is just loafing along. Blew by an old Suburban like the one I used to have (an '86) and it was struggling to make 65 up the same hill. I blew his doors off while getting better mileage in a vehicle that's half again bigger.
Turbo diesels rock!