My 78 300SD started making a lifter tap noise when under light load only. I suspected an injector was failing at first. The noise has progressed and now makes noise all the time. I tried to locate the cylinder making noise by cracking injectors (a long shot on valve noise...but it was quick and easy). No luck. It is definitely a mechanical noise because it does it when engine braking and shutting off the key, eliminating the chance it is a combustion noise. I checked my valve clearance, and the re-adjusted them all. The camshaft lobes look OK to me, no broken valve springs, it all looks OK. I'm convinced the noise is in the upper end. It is loud enough to attact the attention of people I pass on the street, and pretty obnoxious in the car. I really don't want to start tearing it apart to diagnose this noise. Has anyone experienced something similar that might be able to offer some advice here? Any help appreciated.
Joe

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1978 300SD
Running Biodiesel/SVO, 2 tank system
127,000 Miles