[msej449]

It was a replacement BMW M Performance limited slip differential (made by Drexler). Apparently a pretty rare manufacturing flaw: some of the teeth on the gears chipped. But diagnosis took a lot of effort, as it only manifested as a general vibration above 60mph and this meant looking at the whole drivetrain, starting with each wheel - which was removed, checked for damage, and substituted with another then test driven. Once each wheel had been tested they replaced all four but that made no difference. They then replaced the differential and the judder disappeared, and inspecting the diff’ oil, could see a lot of small fragments. So I got a new LSD. To test it had to be driven out of the city to a higher speed limit highway so it could run at 70-75mph, then brought back into the workshop.

So it added up to all that labour and road testing time plus a new LSD.

Vague vibration problems can be very hard to diagnose as once you eliminate obvious sources by visual inspection, you have to systematically work through the drivetrain bit by bit, trying to isolate the cause.