Darsdoug, thanks for the input! That was my thought as well. The chances of valves in both cylinders being stuck seems unlikely. I've also watched the video I posted a few more times, and the amount of hissing might point to a gasket issue (or at least I'm hoping that's the problem). This weekend, the valve cover is coming off for sure, and probably the head as well. Whether it's a gasket or valves, the head has to come off, so we'll get to the bottom of it.