The oil in a modern diff is usually for the life of the car and unless there are problems or oil loss is not normally changed. Perhaps for cheap insurance it could be changed at 100,000 miles or 150,000 km. Up to you but the intervals are normally very long unless the diff is subject to very heavy use or contamination such as may happen in 4WD vehicles that often ford rivers and/or operate in severe conditions.

If you are getting a noise from the diff area it would definitely indicate that there is a problem. It is notoriously difficult to understand just where a noise may be coming from in a Morgan. The fact that it is happening in tight turns may indicate some other point of stress or physical contact, even something as simple as the tail shaft touching the inside of the transmission tunnel. I had this problem in hard right hand turns until I fitted the 5 link suspension.

Peter

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