Do you have access to the FSM? If so, look in the Engine Manual, Mechanical, Programmed Repairs as I recall that there are a couple of possible causes mentioned there:

Timing chain/gear/tensioner
IP out of calibration (differences between injection quantity)

and head-gasket allowing combustion gasses into the oil gallery to the head and causing bubbles to enter the lifters, thus causing lifter clatter.

I have also had a pre-chamber leak that sounds like a sloppy tappet, easy to find by washing a (not hot) engine, then start it and look around the pre-chambers for bubbles.

Good luck. If it gets ugly, I have a known-good IP on my spare 603.960 (was driving it before I decided to part it).

__________________

Gone to the dark side

- Jeff