Have yout tried listening with Mechnics Mtethoscope; sold cheap at Harbor Freight to pin point where the sound is comming from.

Loosen 1 Injector Hard Line Nut see if the noise goes away or changes; tighten it back up and do the same to the rest.
If the noise goes a way or decreases on one Cylinder that is where you are having the problem.
If you find a problem Cylinder remove and swap positions of 2 Injectors and see if the problem follows the Injector from the Bad Cylinder.
If the Bad Cylinder is still making noise you have a problem in the Engine.

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84 300D, 82 Volvo 244Gl Diesel