Need to do a leak down test and that will point you in the right direction..
http://www.carcraft.com/techarticles...r/viewall.html

 

Sort of is a shame they didn't put a couple oz. of oil in there and see it it shot up. Of course I don't know. Certainly does sound like it could be a valve. Great link there^. Makes me want to hook compressed air to it and listen. Anybody ever pull the 02 sensor and listen in the hole with a tube I wonder? Or maybe just another plug hole with it's exhaust valve open.