Thanks for the idea about the heat shield, but that's not it. Holding it down with a screwdriver or the rubber-coated end of a hammer doesn't stop the noise.
I'm still probing around with the stethoscope, but can't pinpoint anything. It's gotta be linked to something I did when I replaced the gaskets for the valve cover and oil filter housing assembly. (Noise started immediately after the fix ... thankfully, no oil leaks or other issues in approx 500 miles, though.) Could it potentially be a VANOS issue? I don't really know what to look/listen for.