Got a new 81 300td. She has 300k miles and is yellow. Shifts good drives good and does good at Highway speeds 70. There's no smoke at start up. At idle it has a knocking /tapping noise. At Highway speeds it goes away. When slowing down it isn't there until it hits idle. Sometimes when I drive to work 38 miles it goes away when I Park. It also idles ruff sometimes.
Had a friend look at it. He said it may be a leaky injector nozzle. (nailing?) so I cut out each injector by loosening the fuel line and #3 seemed to go away. I pulled injector and cleaned it. The noise is still there.
I haven't adjusted the valves yet.

Any help would be nice. I know om603 engines not the om617 very well.

Post video soon.