A guy on the w-body forum also suspected an exhaust tick.
A chain flopping around would have to be hitting something in order to create that kind of noise, and I imagine I'd have more serious issues than just an idle vibration. Then again, I've never removed it so I couldn't say for sure. Part of me just doesn't want to have to deal with removing all of the accessories, draining the coolant, pulling the harmonic balancer, and replacing the timing cover gasket. It sounds like a bit of a PITA. Here'* something I found:
You'll notice in the above picture that the timing chain doesn't have a whole lot of slack, so even if the tensioner is a tad bit loose, the chain would have to be hitting **something**.
But I'll have a listen tomorrow just to be sure, since its getting late to be revving my engine.
The timing cover has never come off. The heads have 85k miles on them, and the rest of the engine has almost 215k miles on it. I can reproduce the sound pretty easily as well. Just rev the engine. The sound from the video is a fairly accurate description of what it really sounds like in person.
The job to replace the timing set (sprockets, chain, and tensioner) would only cost me $50 + shipping, but it just looks like a really annoying job to have to do. Then again, if that'* what'* wrong, I'll have to suck it up and just do it.