Well, I'm stymied.

Tracked the knock to the rear/#5 cylinder by stethoscope, and confirmed by killing injectors. Loosen injector line, knock went away, tighten it, it came back, etc.

After a couple times doing this, the knock went away entirely, with the injector firing or not. I have not been able to reproduce it since.

I'm seriously scratching my head over this one. Long since past time to be taking it to a pro, and I will probably still snap up that spare engine for peace of mind.

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617 swapped Toyota Pickup, 22-24 MPG, 50k miles on swap