The XT headset on the Catalyst had developed a stop slightly off-center a good while ago, meaning it seemingly worked smoothly but centered itself off-center.
It had been over-tightened.
This week I finally came round to inspecting the headset.
I found quite clear marks in the upper cup and some minor marks on the upper race.
The upper race was removed, rotated by a few degrees and reinstalled. This, in combination with some extra careful installation of the rest of the headset, makes for a much better riding bike. Success!
In other news, the XTR rear hub seems to develop a problem with its freewheel mechanism. Coasting and then pedalling produces a loud 'clank' sometimes. And it's getting more frequent.
Is this a common issue?
Are there pawls inside the freehub body. Sounds like one of them is not engaging properly, maybe.