It only showed up warm, only above 4,000 RPM, and only on the right-hand sweeper near my house — which made it very hard to describe to anyone and very easy to start doubting I’d heard it at all.
The forum had three theories within a day of me posting about it: a heat shield clip, an exhaust hanger, or — the one that worried me — something in the valvetrain. The dealer took considerably longer to get to an answer, which isn’t really a complaint; a noise that only appears under specific, hard-to-reproduce conditions is a genuinely difficult thing to diagnose on a ramp.
It turned out to be the first theory: a heat shield clip that isn’t torqued properly at the factory on this generation of car, apparently common enough that the forum knew about it before the dealer’s own technician did. Five minutes to fix once they knew where to look, after five weeks of me flinching every time I got near 4,200 RPM. I’m keeping the thread bookmarked for the next owner who searches for the same sound.