The Audi 3.0 TDI engine is highly praised in Europe for refinement and smoothness (although it's apparently not as fuel efficient as the competition [ BMW + MB]). I myself have experienced Audi A6 3.0 TDI's driving by and you can't hear the engine at all. The new Mercedes 3.0 V6 CDI engine is also a big improvement over the outgoing inline-6 CDI, which was still audible despite being quite quiet for a diesel. Not sure about the new 530d, have not heard them yet. Most folks here drive 520i's.
