It may have just been an un-heard backfire, but would be a good idea to check compression to be sure.

May not be a problem if the engine is running fine.

P.S.: A "Pan-Evac" is a Crankcase Evacuation System. Basically, a 1/2-inch pipe, cut at a 45-degree angle, inserted/welded into your header-collector.

A check-valve is screwed onto the threaded-end of the pipe, and hose connects check-valve to closed-type crankcase breathers.




The pipe, placed in the exhaust stream, draws air from the crankcase, causing a partial vacuum, reducing crankcase pressure, removing blow-by, improving windage. Sort of a poor-mans crankcase vacuum pump. Pan Evac Kit