I suspect a lazy glow plug. An easy test is to check resistance between the glow plug relay connector and each glow plug. Remove the 6-pin connector in the glow relay and measure the resistance of each socket to ground. You should see 1.7 to 1.9 ohms for each. If there is a higher number or zero, there is a bad glow plug or bad wire to that glow plug. The connector sockets are numbered corresponding to the cylinder from front to back.

Is the noise a rhythmic clacking? Maybe you have a worn or clogged lifter.
A possible cause is the head gasket disintegrating which is likely if you see oil leaking on the front passenger side. Eventually the alternator will be grimy. Bits of the head gasket clog the oil passages for the lifters which make noise because of insufficient oil pressure. It can also be caused by improper oil service; infrequent oil changes, incorrect grade and weight oil, etc. Many have cleared the noise by using synthetic oil such as Mobil 1 Turbodiesel and Truck formulation.

The injection pump is driven by the engine. Optimal combustion relies on proper timing of the injection pulse to each cylinder. Over time IP timing can slip because of chain wear and wear of other components.

An Italian tune-up can clear build-up of soot from lazy driving.

Sixto
87 300D