The timing chain of the longitudinal V6 diesel runs at the back of the engine, on the gearbox side, so replacing it means taking out the engine or the gearbox. That alone makes it expensive, but the mechanism is the point: the chains stretch and the hydraulic tensioners lose their pressure. The first and only early sign is a 2 to 3 second metallic rattle audible on a cold start. If that is ignored, the chain jumps or breaks, and the engine is badly damaged inside. The same chain kit also drives the oil pump: if that seizes, the engine is left without lubrication.