Found out what it is, and it’s expensive (in outcome).
Duff pinch bolt on one side.
They were both torqued up equally 4 rides ago when I had the shock and bearings changed (I witnessed it being done). One side was bang in line with Orange’s recommendations last night, the other had lost tension. Not a problem though I, it’s just settled in. Using my torque wrench and checking Orange’s torque setting I slowly tightened the “loose” bolt, shearing it half way down. No damage to the Allen head, and the alloy thread that this steel bolt goes into held up well.
I was being gentle and using a torque wrench having checked the settings, so I don’t think I was being ham-fisted. And if the bolt was suspect it explains why it slackened over a couple of ride.
Anyway, completely inaccessible, other than drilling and getting a screw extractor to it and sheared at a 45 degree angle so the drill wants to slide off.
Looking like a new swing arm minimum.
Ouch.
Like I said it’s done good service over the years. But if I get another one (and I probably will) I might replace those bolts if it ever starts creaking.