Bottom line is: this hasn't come back. One other time I felt the strange idle drop while sitting at a light. I've seen other reports of this on the S54, often occurring more frequently than mine. I'm enjoying the car but paying attention for signs of an issue. I'm seeing none at this point.
I changed plugs AFTER this occurred, coincidentally. I don't know why, but I didn't have them do plugs when adjusting valves. I definitely torqued them correctly, can't say what the shop did before (though I trust them).
As for timing chain, I would like to think they'd have mentioned an issue if they noticed one while in there. Correct me if I'm wrong, but the timing chain is mostly (fully?) exposed during the vanos blot replacement procedure. They did change engine belts at the same time so maybe the noise was a belt wearing funny and settling.
Of course, I barely drive it at all now that its summertime (fall/winter is convertible season here), but it's yet to occur even during a hot day and spirited drive for an hour (there was no correlation to heat or driving pattern before, I'm just stating the few drive cycles it's seen).
My Blackstone report was good last fall during oil change. I might pull a sample soon just to verify things are ok on the inside. Though, this wouldn't tell me that the timing chain is loose and about to slip a tooth and destroy the engine. But I tend to not imagine such things or I can't enjoy my car.
One other item is that the vent hose was not pushed all the way down on the intake side (right side) while at the mechanic. He noticed and pushed it back in until it clicked. I'm honestly not clear what this does so no idea if it could've caused the noise, but I doubt it. Part 2, here
http://www.realoem.com/bmw/showparts...97&hg=11&fg=15