Noise under valve cover
I had the same problem with a 1966 327/300hp Vette engine. The stock valve covers were re-installed upside down after new gaskets. Stock valve covers have a -O- stamped on the flange in the center of the valve cover. This stamped side needs to be installed toward the intake manifold, as the valve covers are not symmetrical. I saw evidence of the pushrod side of the rocker arms rubbing against the valve covers. I hope this solves your problem.
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