Made a little video. I looped it and did some EQ to partially isolate the noise, it is most obvious around 1:43. Almost sounds like a diesel valve train.

That sounds like a rod knock to me.

Problem?
Cause?
Solution?

It did start happening on my way home the other night shortly during/after a WOT romp.

Get a broom handle if you don't have a stethoscope, cut it down a little and use it to listen at the oil pan.

Numerous things can cause a rod knock but the most common I guess would be a loose rod bolt.

Or use a long handle screw driver. Ron is right, loose rod bolt, spun bearings, etc.
Also sounds like cam knock, which the LS series are notorious for.

So probably loose bolt(s) and worn bearings is what I'm looking at minimally. Anything involving the cam means pulling the engine. If I go that far, I'd probably do a bunch of things, like a typical car idiot. . .

Not even worn bearings really, at least not with the cam Dick was telling me that he's seen factory stock engine with as much as .010 Gap on the bearings.

The only things I heard from this engine before were a little piston slap during warm up and a little "typewriter" sound from the valve train. Pretty smooth otherwise. That's why this stuck out.

Listen with a broom handle, stethoscope, screwdriver and see if you can determine where the sound is coming from. I don't know anything about the LS cam issue but if it's a rod knocking it will be the loudest under a heavy load. Since you have a manual trans the only way to load the engine is by driving it. Just get out on the main street and put it in third at a lower rpm and give it some throttle. If it's a rod issue it will rattle loud.

I can tell that's exactly the kind of conditions under which it makes the most noise. Coming home on the freeway when I first heard it I slowed down a bit and tried to cost and just nudge it. Whenever nudging it would get louder, when costing it didn't make any sound.

I'll try to double confirm with a stethoscope that it's coming from the pan area. Assuming that's it, I guess this is a matter of draining the oil (needs an oil change anyway thankfully) and removing the pan to see the rod connections to the crank.