While there is some noise from the injectors (in some gas-direct injection engines you can hear them as well, if you listen carefully) the sound you are hearing in an old diesel, that handful-of-marbles-in-blender-clatter, is due to the more abrupt combustion in a diesel engine in general (only air is compressed, and the fuel burns almost instantly on injection) and imprecise/poor timing of the old mechanical injection systems. Modern common-rail electronic-injected diesels are almost as quiet as gas engines.