Ideas on what's causing my click/tick/rattle noise?

Lots of good info here folks, and thanks. I got off work early today so I'm headed to the shop for round three of "what's that noise". Today's agenda, check again for exhaust leaks, pull the fuel pump and see if that's it, and check the plug wires. As info, the fuel pump was new from NAPA, the plug wires were new from Auto Zone.

AJ, I will check the goodies on your list. I did check the crank endplay during assembly - I don't recall what it was, but it was just fine according the FSM specs. My rockers and arm are in pristine shape with new lifters and valves. I did re-use the factory bolts and washers to install the intake/exhaust. I'm running a Clifford 4-bbl intake with a AVS2 500 on it, and a set of Clifford shorty headers.

One detail I forgot to mention early on, is about throttle position. This noise is far more prevalent under light or partial throttle. If I am accelerating or decelerating hard, the noise is much less than loafing along at a leisurely pace. I have no idea what that could mean but I checked it many times and the results were consistent.