Thanks for the suggestion, JFP. I read in
another thread your comment that it's possible the thrust bearing can fall out, so good idea to double check things are OK here, especially if it's convenient to do so.
Do you think I could get a good measurement on the axial crank play if I just removed the serpentine belt and gave the pulley a good push/pull? Will the timing chains being installed invalidate the reading? How about the clutch assembly? I'm pretty sure the rods and pistons being installed shouldn't invalidate it; probably a good amount of clearance at the piston pins.
This is a 5-speed / manual transmission car, FWIW. If the noise may be influenced by the axial/thrust crank motion, I think I may try as an experiment simply applying a bit of pressure to the clutch pedal under a condition where the noise is audible, and see if that affects the noise.
On another car I had which was known for thrust bearing issues, I recall a very wise mechanic suggested that a good preventive measure was to get in the habit of always starting the engine
without depressing the clutch pedal! If you think about it, it makes perfect sense - loading the thrust bearing with full pressure plate force when you go to crank the engine over, when there is zero oil pressure, must be hard (to say the least!) on the thrust bearing. On that car, (it was a volkswagen) the factory crescent moon style thrust bearings were the best / most reliable. The ones that were most problematic were the integrated thrust/main bearing, especially if the top and bottom halves weren't properly "set" so there was no skew on the thrust surface. It's a bit dangerous to get in the habit of not pressing the clutch pedal, however, if you forget to put it in neutral! (And in the 986, the starter-clutch interlock would need to be "defeated", anyway, so it's definitely not Porsche's intention to start the engine this way). Aftermarket high spring force pressure plates are another factor which can also exhasorbate crankshaft thrust bearing problems...