The low rpm limit most people are familiar with on diesels is a result of the usual American approach with very long stroke and fixed injection timing. The old Mack and Detriot 2-strokes were the worst -- no power below 1800 and none above 2100 rpm, and it threw a rod at 2200. Modern engines tend to run a lot faster, especially the European designs (my Volvo redlines at 5300, the 61x MB engines at about 5000, although you CAN push the turbos higher at the expense of engine life, the 60x engines run more like 4200). The newer American designs are considerably more flexible, too.

The fixed injection timing/long stroke combination works very well for a single speed engine, as in running a generator, but variable injection timing and short stroke (like MB and Volvo diesels) makes for a much more efficent engine over a much larger rpm range.

The noise is the result of late ignition compounded on fixed timing engines by the fact that the injection it much too early at idle -- injection timing is set for high rpm only.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!