The timing chain stretches from 180,000 km, and the chain tensioner no longer holds it properly. The tensioner is hydraulic, and with cold oil at low oil pressure it does not react fast enough: on a cold start the chain goes slack for a moment, and that is when it can skip. If it skips, the valves and the pistons meet. Missed oil changes speed the process up, because dirty oil wears the chain and the sprockets. The symptom has two stages, and they do not mean the same thing. If it rattles for one or two seconds on a cold morning start and then goes quiet, the chain has already stretched but there is still time to act. If the rattling is audible with a warm engine as well, the chain is heavily stretched and the tensioner is working at its limit, and then it is not worth driving on. The chain runs at the rear of the engine, right at the bulkhead, so it is hard to reach, and on many diesels the whole engine has to come out for the replacement. The material itself is not expensive: the chain is 150 to 300 euro and the revised tensioner 80 to 150 euro. The labour makes up most of the cost.