The fact that it briefly went away after tightening the rockers tells me it's an exhaust leak. The valves won't fully close right off, and the explosion in the cylinder won't occur, thus, no noise. When the engine is cold, try "feeling" for puffs of exhaust around the header/head mating surfaces with your fingers. A new gasket and double checking the header bolts periodically should help.
Rev-
FYI: I'm sure you're familiar with the "ignition wrench" set that comes with most Craftsman tool sets, with thin open end wrenches, where one end is normal and the other opens at 90 degrees. Well, the 90 degree 3/8" wrench (or 7/16", depending on your header bolts), is a godsend when it comes to hard-to-get-to header bolts. You still can't torque them, but you know how tight to go. Anyway, that wrech allows for a greater turning radius, lessening the time and frustration that goes with installing and uninstalling header bolts! Dig through your tool box and try it! I know you'll be pleased.
Take care,
-Chris
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