Okay, sorry about the delay, went for another ride today to make some checks after re-raising the post 4-5mm.
As soon as I heard the noise start up (on a hill, as always), I stopped and took off the reflector completely. Rode on, noise still there. The only other thing I could notice was that it's not matched by pedal stroke or the rear wheel (happens in random places of both rotations). The noise definitely goes away as I reduce effort in the pedal stroke. wtf
I think that was a good suggestion to check the roundness,
DannoXYZ, although as far as I can tell it looks okay. Pic (I presume that's what you were talking about?) attached - reflector not.