Exhaust lobe wiped out. Pull the covers and rotate engine while watching rocker arms. You will see if one or more are not moving, or are barely moving. Exhaust can't escape because exh. valve doesn't open, so when the intake valve opens the exhaust escapes into the intake manifold. This is your pop.

There are plenty of known good engine builders that have had camshaft/lifter failures. Lack of good stuff in modern engine oil and/or improperly manufactured cam and/or poor quality cam cores. All though the cam manufacturers won't admit it.