Us a Mechanics Stethoscope or my favorite a piece of Broom Stick applied to where you thing the should is coming from to pin point the sound.

Be sure to check the Vacuum Pump. I the Vac Pump is going bad the pieces will drop down into the Timing Gear/Chain and have cause Engine Damage.

Check the Valve Adjustment for Valves that are too tight.

Check the Timing Chain. If there is enough Timing Chain Stretch and Timing Gear Wear the Valves can hit the Pistons.

Look for loose things that could cause the sound during vibration.

If it has an Automatic Transmission maybe the Flex Plate is cracked.

Maybe the Starter Pinion Gear is not retracted enough and making contact with the Ring Gear.

You might pull off the Valve cover and take a look for any Valve train problems.

I have not read of the Ball Pins making noise when they are loose so cannot comment on that. However, I believe it is unlikely that all of the Ball Pins would become loose at the same time.

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84 300D, 82 Volvo 244Gl Diesel