an update, for anyone w/ a similar issue...
I tried to locate the source of the creak more precisely by pushing against different parts of the shock and mounting points, while compressing the shock. Still had the noise.
Then I rotated the spring 10 or 20 millimeters - voila! No noise. So I...
Comparison video (17 seconds)
I bought each shock new, and they tuned each for me + each bike. It's always been this way FWIW.
Is this much noise normal? A sign of problems?
Thanks in advance!