Is the tick consistent with RPM and constant? Does it appear only when the engine is warm? Only under load?

Ticking and tapping is often valve train, usually lifters but a sound likened to a diesel might be less constant/consistent and lean more toward a timing/vacuum/fuel delivery issue.

One of the sweetest purchases I made for my tool box was a fairly basic mechanics stethascope for under $20 (not the crappy $7 one from Harbor Freight). A long screwdriver works to if you place the tip at the suspected noise source and the handle firmly on any bony prominence adjacent to your ear, It tends to relay a great deal of sound and may help you isolate the tick much more effectivly. I thought I had a bad lifter on another mark inline 6 and with this trick I found that it was just a bad injector.

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