Some years ago (2018) on a long European trek in Spain/France, we had a rather similar sounding "clanking" noise from the rear that was related to rear wheel rotation. It started after some big climbs in the French Alps. We had a serious look around but could not locate anything obvious wrong - we were however limited in what we could do while on the road. We cut the trip a bit short (missing the Grindelwald gathering) and made our way home taking it relatively easy. As we approached northern France, it stopped ...
Once home we dismantled the rear end and found nothing obvious ... frustrating ... but it is a Morgan
Conclusion/suspicion is that it was something knocking in the rear drum brake. We have not had any recurrence but a lot of recent miles have been with a replacement disc brake supporting a hypothesis that the original drum might have been the source...
Good luck to PJH for a successful resolution.
Those are the stories that scare me, as I'm very anal about mechanics and understanding them. I hope this doesn't send me on a wild goose chase, but we'll see
