What is this engine tapping noise?

I have a 93 Chevy Blazer, 4.3 Vortec, Vin "W" (Central port injection). It's a balance shaft engine. The engine is making a tapping noise, sounds exactly like someone tapping the tip of a pencil against a table. Most of the time it's steady, but it gets louder and softer (sometimes it entirely disappears for a few seconds). Also, sometimes it turns into a double tap, like someone tapping both ends of a pencil on the table. When it does that, the sound seems to move towards the right side.

The tap seems, for the most part, to be in time with one of the cylinders. I've read that pulling off a spark plug wire will make the sound change if it's piston slap in that cylinder. I tried it for each cylinder, got no change but on the #1 cylinder I was getting shocked in time with the tapping sound.

The sound seems to be coming from low on the engine, it is definitely not in the valve covers. When I listen there, I can hear the tappets, they're quiet and smooth. It's also in the front of the engine, right around the crank pulley, not back in the oil pan. It is not an engine accessory, I've tried running it with the serpentine belt off. The crank pulley has some visible wobble when the engine runs.

Other info: The oil pressure reads high, tops out the gauge most of the time, but fluctuates (sometimes comes down to 30) once the engine is completely warm. I'm running 10w40, should have used 5w30. I did a motor flush before the last oil change, the oil appears perfectly clean. There is no significant oil leak (engine is dirty, no drips). The engine is running fine, not missing. The noise does get faster and louder with rpms.

So, does anybody out there know what this is?