I found this interesting:

The two main factors that held diesel engine back in private vehicles until quite recently were their low power outputs and high noise levels, characterised by knock or clatter, especially at low speeds and when cold. This noise is caused by "piston slap", the sudden ignition of the diesel fuel when injected into the combustion chamber slamming the cold-contracted piston into the cylinder wall. The tolerances between the piston and cylinder wall are greater at cold temperatures to allow expansion at higher temperatures. A combination of improved mechanical technology (such as two-stage injectors which fire a short "pilot charge" of fuel into the cylinder to warm the combustion chamber before delivering the main fuel charge) and electronic control (which can adjust the timing and length of the injection process to optimise it for all speeds and temperatures) have partially mitigated these problems in the latest generation of common-rail designs. Poor power and narrow torque bands have been helped by the use of turbochargers and intercoolers.



http://en.wikipedia.org/wiki/Diesel_engine


>>Alan, I've noticed the new common-rail diesels have a characteristic clatter at idle. Quiet, but different from a petrol engine. Does yours do that?
>
>I think every diesel clatters a bit when idling - don't know what makes it do so, but I guess it's just that it's idling at lower RPM than your gas engine. I never had one, but drove half a dozen of them in the nineties, mostly office cars. A friend of mine claims 3.7 l/100km in his Peugeot (304D, I think). Sort of "don't forget to remove the spider web around the tank top before fueling". Nope, tank top is something to wear. Tank cork? Never saw one made of cork. The whatever you use to close the tank intake tube, OK?