Where the injectors properly pop tested and balanced? Although I suppose that'd go away with the injector line cracking.

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1983 240D 3.0T 4-speed manual, now sold

1989 Subaru GL Wagon 5-speed Touring Edition

They were not but its not an injector noise; it is clearly mechanical. And you are right, the sound does not go away with opening any of the lines....

Would it be of any benefit to pull the oil pan before tackling the head, to see if there is anything in the pan?

Pull the injector and look in the hole. The ball in the prechamber could be broken or the end of the prechamber could be burnt off. Pull the prechamber and look in that hole it is a lot easier than the head to start. You should not be able to get a straw into the cylinder past the prechamber end the holes are radially out. Check the vacume pump bearing.
I have a small led light that will go in the small holes so I can look around.

Hope this helps
Robert

ok will pull the injector and the pc...