Why so quiet?

I remember, in the early 90's, when you went to a ford, chevy, or dodge dealership, the only way the salesmen would let you start and testdrive one of their shinny new diesel trucks is if they checked over thier shoulder to make sure there wern't any tree huggers or old ladies with pacemakers around to hear and see the wonderful sights and sounds of the oil-burning workhorse you just kicked to life. Or even back in the y2k scare when you could hear any cummins or powerstroke from three miles down the road, and be able to tell what year the darn thing was. (Now I'm reffering to stock trucks.) Yesterday, I walked outside my shop to admire a friends brand new '09 F-450 king ranch, and didn't even realize the flippin' thing was running until he popped the hood and I saw the fan spinning and the faint sound of an ethiopian sweat shop. Don't get me wrong, the truck is 100% badass, but it's just SO quiet. I started asking around, "why?". What exactly is it that makes the most powerful current diesels so much quieter then those of yesteryear. I've got several theoris but no one has a clear answer. So anyone got some light that they could shed on this subject?  

Well, there are several answers. For one thing, the vast majority of the newer diesels employ what's called "pilot injection" which basically means, instead of just one injection pulse, there are several smaller ones, which results in a more gradual pressure spike in the combustion chamber. The rapid increase in cylinder pressure was one of the major components in the traditional "diesel clatter."

Secondly, a lot of the newer diesels have a lot more sound-deadening material around the engine and passenger compartment. But like you said, even with the hood up, they're relatively quiet. I'd chalk most of that up to the pilot injection, although generally, the OEs have gone after noise on diesels pretty rigorously. Although it's not as much of a factor with trucks, I think the idea was making them as quiet as possible to appeal to a broader audience, and prove to them that diesels weren't the clackety old tractor engines they used to be.