It's 'dieseling'. /w3tcompact/icons/wink.gif Seriously, though, that's the truth. Gas engines make the same sound, though it's a lot 'tinnier' because of the lower mass of the block and heads, when the fuel octane is too low or there are carbon deposits in the head because the fuel is igniting before the spark - so it's called 'dieseling' or 'spark knock'.
In order for diesel engines to complete the combustion process before the piston finishes its downward stroke, the fuel is injected just before the piston reaches the top of the stroke. The combustion process begins immediately, even though the piston can't start down yet, thus the "knock". The clatter is usually worse at idle because the pistons are traveling slower, so more of the combustion takes place before the piston starts down. Some of the new engines with electronic valve and injection timing greatly minimize the clatter by delaying the injection of the fuel until exact top dead center at idle, and slowly increasing the injection timing as the engine rpm increases.