Even the sound of the newer engines is reduced. Common-rail fuel injection reduces injector noise, which is most of that rattle you hear. The emissions equipment muffles the exhaust a bit. Even valve noise is reduced because they need to control things to finer tolerances.

But a big diesel will always sound a bit beefy I think. The smaller 4-cylinders shouldn't be discernable from gas at all.

My 2.0L turbo diesel engine (on non-VW variety) sounds very different than its 2.0L gas sibling. And I definitely like it :)

Our now-maligned cheater TDI really doesn't sound any different than other 4 cylinder engines of its size and power. The exhaust does smell different, I prefer it to gasoline exhaust (don't _like_ it, but find it less offensive).