The "soft rattle" (a gentle thumping) occurs most clearly when I tip the lens up and down (front pointing to the sky or ground). It doesn't matter if the lens is attached to the body or not. It feels like a heavy assembly inside is shifting a bit., and you can almost feel the momentum of something heavy inside moving as the lens is tipped. The soft rattling/thumping occurs whether or not the lens is powered up by the body.
I decided to ask the forum even though the lens seems to work perfectly because I don't hear any noises when the 21, 30, 45P, 65, 90, 120, or 135 XCD lenses are similarly handled. Of course the 80/1.9 is a more complex and heavier lens than just about any other XCD prime, so I assume this is a normal aspect of the design, but thought I would check.
I should also note that other lenses from other manufacturers also show a very similar soft clunking noise when tilted—most notably, the Sony 135/1.8 GM among the lenses I've used recently. I vaguely recall that years ago when I was using large Nikon and Sigma lenses with optical image stabilization (vibration reduction), some of those lenses made similar noises, though of course neither the 80/1.9 XCD nor the Sony 135/1.8 GM have optical image stabilization.
Could any other 80/1.9 users out there compare notes?