With your suspected Diesel Leak onto the Belly Pan I am thinking the Fuel Injection Pump O-rings might be leaking.
Next thoughts would be the Hydraulic Valve Lifters ticking or the Vacuum Pump.


Harbor Freight sells cheap Mechanics Stethoscopes or I use a 3 foot piece of Broom Stick Handle with an Ear on one end and opposite end making contact with what I want to listen to. Watch out for Hot and Moving Parts and find out where the Noise is coming from before picking something to fix.

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84 300D, 82 Volvo 244Gl Diesel