[lrf]

I would check that manifold and gasket again, then try a "sounding stick" placed to your ear and along the valve chamber; timing gear cover, and finally the oil pan. Also go to the tailpipe and listen, this will tell you if it's in the valve area. Our club engine had a knock which turned out to be caused by the oversized .004 distributor/oil pump drive gear. I learned this stuff from the "old timers" in our club which suddenly, I'm now one!