Do a search for some of the threads; it is a frequent comment. Ask one of the folks that had the same problem if it cleared up for them.
I have always wondered if people got used to the softer sound while the "O" rings were leaking and now that the leak is fixed and the injectors are getting the full amount of fuel they are not used to the louder sound. But, I have not read a conclusive reason for them being louder.

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84 300D, 82 Volvo 244Gl Diesel