If these earlier L.T's.O.P. were set up right, there should be very little discernible sounds from under the valve cover. Agree that a possibility of crack exists - as does leak at the flange itself, but, such an anomaly should produce noise at all engine speeds and loads. Another tidbit about the valve-train is that the (OEM) springs were very 'weak' - ie. low seat pressure with valves closed. With .375" valve lift, the pressure was only rated about 144 lbs. - compared to 318's rating of 177, (2Bbl) 360 of 208, and 340/360 HP at near 240 lbs. ALL OF THESE springs are interchangeable. My own stock springs, with 45 years of use on them were between 125 and 135 lbs. so I installed a matched set of 318 springs. Even with low-revvers, (like most stock sixes), valves not closing 'promptly' is something to be aware of - which can result in burned valves. Now, this leads us into one more top-end noise possible cause and that could even be a leaking intake valve. Although rare, it would produce more of a subdued 'popping' sound in the intake manifold - as load increases. We have no idea how many miles on O.P's engine so all I can suggest is to continue trouble-shooting and eliminate causes, one by one. Oh, and a compression test would also be a good idea. Good luck.