Could just be a lifter. If it is, it's not really worth worrying about. Swap the injector if you must, but check the others with your stethascope first. If they all make the same sound, no use changing them. IIRC, if something was in the prechamber, it wouldn't fire correctly and there would be a thumping sound, not ticking.
If it's a lifter, one Mercedes tech I talked to said that it wasn't worth the repair, unless the engine was already apart, and that not replacing it wouldn't cause any damage. Just my 2 cents.

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'95 S320 LWB
'87 300SDL
'04 E500 wagon 4matic