On Sep 11, 2023, Allen Frechette in Auburn, NH, USA wrote:
"A loud tapping sound seems to have developed during the first few hundred miles on an engine that was rebuilt back in 1989 and then sat until this past spring. Runs amazing, good compression, no leaks apparent, replaced (again) exhaust manifold gaskets, ran a smoke test on the exhaust, double checked everything I could. Sounds like an exhaust manifold leak, but perhaps not"?
YouTube video (for as long as it lasts).

Followed suggestions that it may not be running on all four cylinders, or running on one carburetor only, a cracked manifold, a loose gasket joint, broken valve spring, misfire.

"I ran a compression check on all of the cylinders last night, and they were all even and at 155-ish. It feels a bit lumpy".

On Sep 12, 2023, Allen Frechette in Auburn, NH, USA wrote:
"Checked the plugs (pulling one at a time as usual) and they all had proper response. ... NOS cap and rotor, new points, and the timing (approximated) at 30 degrees total. ... It doesn't feel like it rev's up smooth. Plugs look nicely brown. I set up the carbs with an 02 sensor air fuel gauge. It idles nicely, sounds ok up top, but under acceleration or rev up, not so much. ... Not sure about the valve adjustments, but I can't imagine it's a backfire or detonation through overlap coming through a carb"?
More YouTube video (for as long as it lasts).

Suggestion: Valves are way too noisy. start with a dive into the valve train, adjustment and inspection.

On Sep 13, 2023, Allen Frechette wrote:
"As it turns out the rear most exhaust valve had loosened up. Very strange, but as I recall I didn't actually have to readjust any of these prior to starting the engine for the first time. ... I readjusted all of them to .015 and it sounds much better.
More YouTube video (for as long as it lasts).

On Sep 18, 2023, Allen Frechette wrote:
"And now, the rest of the story! After a shakedown run over the weekend, the rattle came back? I removed the valve cover and noticed the same rocker was again, loose? Then I noticed that the whole top end seemed to be dry? I pulled the rocker shaft and observed that the gent who built the engine put the towers in the wrong place, so the shaft wasn't getting oil. I am pulling the engine and transmission just to see if there are any other surprises".

Look at the illustration below. Notice the rear pedestal has a different part number from the other pedestals. The rear pedestal has a vertical drilling to pass oil from the cylinder head up into the rocker shaft. There is also a top plug (with locking tab) that serves as a keying set screw to assure correct orientation of the rocker shaft, keeping oil feed holes in the correct position. If any of this is misaligned, the rocker arms will not be properly oiled.