Worked out the weird noise was a bearing, but wasn't the alternator.
The bearing on the fan pulley had died, as had the fan clutch. And the passenger side front wheel bearing...
So finally going to install the twin 12" thermos I've had sitting here with fan shroud. I'll see how they go with the shroud, but better than no fan, and if they don't perform well enough I can always go back to a clutch fan. Seemed silly not to use the fans I had on the shelf for now and give them a go, when the alternative is buy a new fan clutch, and probably need a new fan shroud (mine looks pretty sad).
At the same time I'm going to delete the offending bearing. I've removed the fan pulley and instead of replacing the bearing, at the moment I'll leave it off, put a spacer behind the alternator bracket, and run a shorter fan belt.
Should be an interesting test, see what difference it makes if any. Winter is a good time to test it, that's for sure.