Something is rattling pretty good. Have you checked your heat shields and exhaust components for loose stuff? I'm guessing a heat shield near the manifold and or primary catalytic converter. At least, that is my hope. Otherwise, it really sounds like it could be a problem with the exhaust valves and I would hope it's not that. You get regular oil changes or just top it off every now and then?
If you have a current warranty or extended warranty, I think you should bring it in.