Thanks Larry, Alon, & Q for your insight and prompt responses.
Well,,,I've agreed to pick up the car this afternoon- unresolved- with credible assurances from our Shop Foreman that he'll continue to research possible causes by tabling the case within his regional meeting roundtable, Monday and get back to me.
I did bring up the timing chain/tensioner as a possible source/cause and he responded verifying with his observation that the clatter appears to be coming from the back-firewall area, reducing the possibility of this being the culprit (non-proximity).
Q- the hydraulic issue (oil galleries/ passages, oil filter/ by-pass valve, etc.) was raised and kicked around in light discussion also, but there does not appear to be a test to verify that this is the failure or cause (beyond replacing parts at random or tearing the engine apart?- - & I'm told this is not a real option).
The shop foreman also proposed this explanation: in his experience that some diesel's have similar design lifters and that this condition is "par for the course" on these diesels (remedied with by revving to 4000+rpm at initial start up for a few moments).
I'm curious if 85,000+ mile M104's with these lifters are exhibiting similar unresolvable tapping/ clicking/ chatter in mass?
I'll keep us all posted.