1985 300TD 315,000 miles

My car runs really well. It starts instantly, has good acceleration, burns less than 1/4 quart between oil changes and has been meticulously maintained since new with all service records.
Lately the car seems unusually noisy at idle. Not any weird noises, just a louder than normal diesel running sound. As soon as you raise the idle from 750 up to 1000 rpms, its sounds like a normal running MB diesel.
Has had recent motor mounts, valve adjustments etc etc. Oil has been changed every 3000 miles since day one.
The only record I CANT find is a change of injectors. Could bad/old injectors cause a noisy idle? It is not a "nailing" sound....just louder than normal diesel "clackity-clack". Could these still be the original injectors after 315K miles? Any other ideas on this?
John

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1985 300TDT
2013 E350 Bluetec