Hi all, 68 camaro 350 ss automatic trans. When i drive around 20 mins and reach around 70ish mph 3500 rpms on highway i start to get a rather startling popcorn popping sound from the engine bay, had valves adjusted i noticed it affects valve adjustment when it happens. It starts fine and runs fine at lower speeds except maybe extremely light tapping from the valve or lifter area on the front left side. The popcorn sound seems random and gets worse if i do not let off and pull over. 20 mins after pulling over i start her up and it starts fine. Exhaust header gasket leak? Pre ignition? Exhaust valve leak?Do i need to run premium all the time ? Octane booster?

 

...first place I'd start is running premium... Run some sea foam through the top end too and see if those 2 things help any. Sounds like detonation issues...