Just a little write-up here in case anybody has a similar problem.

Driving to my girlfriends house the other day my engine developed a sound that was identical to an exhaust leak. I ignored it until it changed from an exhaust leak sound to one of those very bad "my valve train has gone down the tube" knocking sounds. Once at her house I checked thoroughly for exhaust leaks and found none (so no headers for now... .) My girlfriends dad suggested flushing out the engine with with Gunk Engine Flush. I've never put anything in an engine besides oil, so we argued about it for 20 minutes and I finally gave in. The oil that came out after the flush was terrible, much worse than with any other oil change (it was about 4K since the last oil change.) After putting in the new oil the sound was gone! I've driven about 200 miles since then and I've noticed that the engine idles and accelerates much smoother than before. I'm hoping it was just something in the head that was gummed up, and I haven't covered up a greater problem here.