When was it last serviced? Have you noticed anything different about the characteristics of the engine such as less power, less responsiveness, unsteady idle, more exhaust smoke etc? First thing I would be doing is cracking the bleed bolt on top of the fuel filter housing and checking that you have a good clean flow of diesel without air. Next thing would be a full engine service. New oil, oil filter, fuel filter and air filter. Also check intercooler and air intake hoses for leaks or internal collapse etc.
-Tom