Still trying to puzzle this one out.

I listened with a long screwdriver all over the valve cover with the engine running. The noise has seemed all along to be coming from the intake cam area on the back of the head. Took a listen to the injectors, and the noise is loud there, but no more or less loud than on the cover along the intake cam.

Yanked the injectors anyway. Nothing visibly wrong with them. Screens were all clean.

Wrapped the injectors in socks. It doesn't cut down the noise.

Now that I've calmed down about running the engine with a rattle, I have run it for long enough for the oil to heat up. That drops the idle to normal, and the engine does *not* make the noise below 1500 RPM. In fact, it's very rev-specific once the engine is warm. Needs 1600RPM and not one RPM less before it will make the noise.