The latest in troubleshooting:
The sound seems to be influenced more by the accel/decel mode of the engine than anything else. The sound seems mechanical and in sync with the engine speed, whether it's half/same/double speed I'm not sure.
Next step: change the oil filter and add the new oil then run the engine again. An old timer at the NAPA store has heard a lot of engine noises over the years, but the fact that the noise is louder when accelerating and quiet when decelerating left him scratching his chin.
As for a broken Piston, another fellow had three broken pistons in his V8 and they didn't make any noise. From what I could see of your Pistons and Rods, they seem OK.
The next test I'll try is to check the valve lash, The noise doesn't seem to come from there, but... Depending on the result I'll remove the three V-belts and run the engine for a half a minute. That should eliminate the Vacuum Pump, etc.
The engine has never had a full rebuild; it has 66k miles on it. It was out a few years back and the valves were reseeded, new piston rings, new timing belt (and a couple since then), probaly some other items I am forgetting. Last year the engine was out for a new exhaust manifold.
I replaced some of the AC pieces last year and have not noticed anything out of the ordinary regarding heating and cooling.