1979 Chevy Cheyenne popping noise

Take your valve covers off and check for a broken/bent pushrod or a cam lobe gone. The best way to test for cam lobes is to start teh vehicle and look for a valve that doesn't operate like the others, like it won't go up and down as much. Then you can check for a bad lifter or cam. Its 'probably your cam so you'll need to replace cam and lifters. Make sure to lube them with graphite grease before starting the car.