N47 diesel engine that has a history of problems with the timing chains and chain tensioners. I ran one of these engines in a 320d ED a few years ago and it doesn't like repeated short runs from cold starts especially at low temperatures and this can promote active regeneration. Apart from all the usual things, the signs of this at the end of your test drive will be a hot smell when you get out and possibly the radiator fan continuing to run after the engine is switched off.