Good suggestion. I checked that. I removed my Ultegra 6500 12-23 9 speed cassette this morning. I had some rust on the rivets from the rain ride earlier in the week. I cleaned the titanium freehub body on my Dura Ace 7700 freehub and it was a bit dirty with a few silvery metal bits visible in the old grease but the splines weren’t chewed. I buttered on some Finish Line creamy Teflon grease (light coating) onto the splines then re-installed the 6500 cassette. I torqued the lockring to 29 ft/pounds. It is very secure - I would say low likelihood of the noise originating from this source.

I’m using DT-Swiss (Giant branded) ratcheting skewers which originally clicked when I failed to tighten them up enough. AFAIK they are now plenty snug though I suppose there is still a very slight chance that some clicking could be resulting from their use.