New noise from a new engine

I'm using early 340 manifolds on '86 318 heads, and the air injection holes below the exhaust ports have been plugged.

A stethescope didn't tell me anything conclusive externally, neither did pressurizing the exhaust pipe. The noise still varies in intensity and at times sounds more metallic, making me think about a failing lifter. They're the original hydraulic rollers. I'm going to remove the valve cover(s) and check for loose pushrods, as well as probe further with the steth.