E31 Hydraulic hose failure
The horrible sound of a whining power steering pump alerted me to suspected low power steering fluid and indeed the area around the right side of the engine bay was very damp with a lot of fluid around the steering gear itself.
I put some paper above the steering gear and under most of the hydraulic pipes to try and narrow down whether the leak was the gear (very expensive!) or somewhere else.
And eventually found this very unusual hose failure – it’s directly under the brake master cylinder so you can’t even see it from above. Very fiddly to change out.