I'm unsure what it might be but to help eliminate things... Is this happening when you lift the bike from the saddle, main frame, rear section.
Main frame, top tube or seat tube. It's not the saddle/dropper. It doesn't do it with the shock removed. If I loosen the top shock bolt a bit, it exacerbates the same noise/feeling. Tighten back to torque spec lessens it, but it's still there.
@TooTallTim started
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I'm unsure what it might be but to help eliminate things...
Is this happening when you lift the bike from the saddle, main frame, rear section.