Well, enkeivette came over tonight and we took a listen. He thinks, along with Guy's original estimation and I agree with the logic of it anyway, that it probably is cam knock. The sound is most concentrated when listening with a stethoscope on the low back part of the intake closest to the block.
Not sure what the next step is, maybe change the oil for some heavy stuff and see if it's better or the same? Have Ron, Guy, or Paul come by and take a listen.
NOTE: While it's clean as a whistle underneath, something is consuming oil, we had to add a couple quarts. That was never a problem before, so I don't know if a couple rings have some gaps that are getting blown by as well.
So maybe a leakdown test is in order.
I'm half tempted to just pull it and go through it, replace all bearings and swap out lifters and rockers and check pistons, replace rings and hone if needed.