Before taking it apart, I ran the car to try and localize the knock. Found it to be coming from either #3 or #4 cylinder towards the firewall. The air cleaner bracket looks to be relatively new and is not the culprit. Nor is the alternator (sometimes they can make knocking noises?). I still have not checked the vacuum pump but am fairly sure that isn't it. The sound is coming from the top of the engine. The cam lobes are in GREAT shape and nothing is physically loose in the valve train. Interestingly, the last owner had replaced the timing chain but never said if the noise started after that or not.
As it turns out, I don't have a socket large enough to fit the injectors so they'll have to wait until tomorrow. I'm quite curious to see if i find anything in the pre-chambers.