I think you might be able to check it. If this Vacuum Pump has a Piston it is claimed that when it holds Vacuum that the Piston and the Parts like the Arm and Bearing that actuate the Piston are pulled away from the Timer Cam.

If that is so you should be able to detach the Main Vacuum Line at the Vacuum Pump and seal that Vacuum Pump inlet. When you start the Engine if it hold Vacuum like it is supposed to the Tapping will go away.
When you un-block the Vacuum Pump inlet then the Tapping should resume.

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84 300D, 82 Volvo 244Gl Diesel