Having spoke with my tuner he is inclined like me to believe the pressure is likely coming from boost going through the intake ports into the head via the head ports.
I will likely be trying to plug my head ports in the coming weeks. Not too hard of a job it seems. I can get the intake manifold out in about an hour, and from there it's just basically a standard valve cover gasket job plus an hour for plugging the ports.
Good Saturday or Sunday job while my girl is at work
