I haven't had time to follow the above link but I will tell of my experiences here.
I have seen this a number of times and have discussed it more than once with other shops. Figuring you don't have a compression, timing, or injector problem, let me give you a theory. The quantity of fuel delivered is the quantity of fuel pushed by each individual piston stroke into the fuel line. The fuel in the line being non-compressable pushes the same quantity out the injectors. There may be numerous strokes between the time any one parcel of fuel enters the line and when it finally comes out the injector. The column of fuel thus must stay in the line.
In between strokes the fuel stays motionless in the line, checked at each end; one end by the injector the other by the pump element check valves. These valves are sealed internally by some brass o-rings. The leakage of fuel either through the check valves or the brass seals reduces the fuel in the line and affects the delivered amount.
Now for the history. I have, and at least two other techs have, changed the sound of this nailing, by retorquing the fitting at the base of the line. I have made the hypothesis above. It is also based on many idling problem repairs in all MB diesels by replacing these seals. The noise level in the 602/603 motors is only a hypothesis.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician