If it's a 'pop' (or perhaps a clunk) sound when you are steering the car at full lock, say turning around in a car park, then it's probably the same as mine. I've looked into this and it's caused by an air bubble in the power steering pipe. It happens on some cars and not others but it was one of the many things I had BMW GB extensively investigate and it's not fixable (unless you replace the power steering system). But the good news in my case is that it's not dangerous. Get yours checked and suggest this to them and see what they say.

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