Aside from the normal diesel growl there is a ramdom light rattle/tap at idle, also a very slight lope - lope is inaudible and almost indetectable by feel. if I boost the idle a little this tapping speeds up with it but remains inconsistent. At ~1300rpm it becomes perfectly smooth and has only the normal mild diesel growl and devoid of any strange sounds, just smooth sweet and eager sounding.

Im thinking if it were a rod, lifter or a cam bearing or something it would get louder as RPMs increase AND it would be more consistent, this tap Im attempting to describe does not sound with each stroke but only when it wishes and in varying intensity from moderate to very mild. Think marble in a bottle. The long screwdriver against the block and head to my ear netted nothing but the normal smooth clickclickclickclick fairly consistent from front to rear. If I had to try and isolate the sound its from the top half of the engine and maybe center or frontward but its hard to isolate.

Does this sound like an injector firing to soon or to much?

This engine has Hydraulic lifters, correct? (87 603 300SDL)

Hard part failure? most of my trouble shooting is with gasoline engines and Im fairly new to MB diesel. Any pointers or input welcome. Milage is unknown, vehicle history is subjective. This engine feels really good aside from this sound.

TIA for responses and pointers. I have to proceed carefuly as cost is starting to become an issue.

Thanks folks!!!