Originally Posted by JohnZo

Pulling spark may isolate noise from a piston/connecting rod, but not the valves.

Engine parts expand when heated, contract when cooled. My engines sound off for several minutes after a drive. I think the interface between the aluminum exhaust ports at the head and the cast iron exhaust manifold is the main source, since aluminum and steel expand at different rates. So, it could be an exhaust leak develops when hot, but is fine when cold. I have multi layer steel gaskets now, but wonder if fiber type exhaust gaskets would be quieter.