Originally Posted by Woodsman
As I understand it, it is mechanical noise due to it being a mechanical injection. The ultra-high pressure injectors have played a big part in quieting the new diesels. However, if you want simple and reliable you want mechanical.
I have been told that the reason that Toyota's diesels are so quiet and Detroit's are so loud is that Toyota uses indirect injection (precups) and Detroit uses direct injection. I do know that my 1HZ was REALLY nice and quiet, and unlike a powerstroke or cummins, you could actually hold a conversation while standing next to it while the engine was running.