Car went in for a health check Saturday. Pleased to say it got a clean bill of health other than the minor little items I am working through at the moment and already known about.
As for the chatter, well they could find nothing wrong. All bearings had acceptable levels of free play if any and a test drive, visual and full diagnostic check revealed nothing else of concern.
Their best guess was the heatshield making contact with neighbouring components when cold and they were able to replicate a similar noise by emulating this. This was in line with my thinking but there needed to be a fair amount of movement of the heatshield to make contact so I am not sure. In any case they have bent the heatshield back a little and i will monitor.
Major caveat here is that the car was of course really warm when it arrived with them but they did check anything and everything they thought could make a noise such as I described.