So I tried coasting without a chain: still noisy. Also, I provided an erroneous observation in my first post: the noise is still present if I am not on the bike, just less noticeable.

I think I've narrowed it down to the cause, though not the solution: the noise appears to come from the cassette rattling around on the freehub radially. To clarify, the lockring is tight, there is not axial play. Yet, grabbing the large cog with my fingers I can easily move it around a couple mm radially. A bit less noticeable with the smaller cogs.

Cassette is shot? Tighten the lockring even more? Live with it?