More info from the exhaust cam:

On this cam, only the beltside bearing cap had been up. I lifted this one and looked underneath and, alarm bells! There are little black flecks in a section of the bearing surface.

Not major damage yet -- they can't be felt with a fingernail, but I am going to polish them out carefully before the bearing goes back on. But how did they get there? That cap was cleaned and re-oiled before going on. And all of the flecks are on the outer part of the cap, towards where the cam seal should be.

What? Let's check out that new cam seal. Ah. There's some dark material around its edge. I don't have the cam out, so all I can do is to edge the seal around in its bore. There's oil down in the lower half of the seal bore -- could I have missed that when I put in the new seal?

OK, here's my current guess. The lower half of the seal bore apparently had some spillover oil in it when the new cam seal went in. This could allow the seal to squirm and flex and move under rotational load when the cam began to spin.

The edges of the bore would then have shaved off some little bits of seal material and let them get into the bearing. Where they would have made these small marks, and made noise.

What I'm going to do now is to clean the bore, RTV the outer edges of the seal to prevent any further motion, polish the bearing, and reinstall everything. That won't get done until tomorrow. Will update when I am done and tested.