PEH:

The easiest and cheapest is a full bottle of RedLine or similar in a full tank of fuel. If the problem gets better, then returns on the next tank, you have an injector problem, most likely. You don't have to take anything out, etc. for this.

Next best is a nozzle replacement (or just switch injectors around) -- if the knocking is gone, the nozzle was bad, ditto if it moves to another cylinder. While you are at it, check for a broken ball pin or bits of burned out injector in the prechamber -- either will cause injector knock, as will installing the seal upsidedown or putting a new style seal on top of an old style seal (I don't think this is the case, your car is too new). There is a faint chance that a burned out glowplug is lowering compression some, too -- the Champion plugs I took out of the Volvo had a hole in the end were the filament separated from the steel tube, making them hollow.

If that doesn't do it, compression check and leakdown test are in order. You will probably find a compression leak of some sort on that cylinder, and repairs will be determined by what is wrong.

If compression is fine, leakdown indicates no problems, your IP is bad. Might only be a leaking pressure valve holder, might be a plunger/sleeve problem, if new pressure valve seals don't fix it, it's Bosch injector service time.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!