That sound is the small gear jumping cogs on the larger ring gear. The teeth are too small and there is too much clearance/flex in the mechanism as well as the little nibs that hold the alignment are not adequate, just a poor design/poor parts quality overall.

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1988 325is (purchased new) sold
2004 X5 3.0 2005 X3 2.5
2008 X5 3.0 (new to me)