If there is pressure leaking then that probably will be the sound. I doubt very much its as made as your anticipating.

Does this engine have the cast iron head still like the earlier HCS pushrod engines? I had one of those and i literally almost dragged the car across the drive removing the plugs cos they were rusted in.

SOAK them in WD40 a few hours before and use a nice long breaker bar to get them out!