A vacuum test may show low manifold vacuum- Sometimes.
If so check for pluggage in the exhaust.Sounds like it was parked long enough for serious internal deterioration of the muffler, and the ever present "varments" love to build nests in the pipe--looks just like a mouse hole.
Loosen the head pipe enough to get a sizable leak and see if the popping goes away.
Pull the valve cover and lube the valve stems and other---.Sticky valves/guides can vary as the engine warms up causing the valves to not seat quickly--maybe even throw a pushrod out.
