Please post a comment if you have any insights!
It becomes quieter once the engine warms up. I've bled the lifters multiple times, and they all seem to be in good, solid condition. Definitely sounds like it's coming from the head, but I can't be sure - hope it's not piston slap? Perhaps the valves need to be re-shimmed?

The bottom end is rebuilt with forged pistons, and each cylinder makes ~160psi compression. Replaced water pump, oil pump, cam sprockets, chain tensioner and chain guides.