Pulled the belt when the car was warm and can confirm I still get the noise, so it's not pulley related.
I did a smoke test and was getting some whistling around the center oil fill tube joint. I'd already addressed an oil leak here and replaced the clamp with a worm-drive style but I sealed this off with some Curil T2 to fix the minor vacuum leak.
Still getting the noise but I
finally found some other examples of the same thing. Three videos and two threads about it - all of them with a cheaper aftermarket exhaust with an x-pipe.
https://www.youtube.com/watch?v=s9poqfZ4m2I
https://www.youtube.com/watch?v=-N0gG2d5grw
https://www.youtube.com/watch?v=gXju8ZJfySk
http://986forum.com/forums/boxster-general-discussions/28505-strange-exhaust-noise.html
https://www.986forum.com/forums/performance-technical-chat/21874-what-sound-exhaust-question.html
Good think I kept the stock exhaust around
