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You are talking about the serpentine belt that drives the alternator, or the one that drives the air conditioner compressor, correct?
This can be caused by a missing bushing or a loose bolt on the alternator or compressor or power steering pump. It is more likely a failing tensioner or tensioner pulley bearing. On this car, BMW sells the tensioner assemblies complete with pulleys. They don't sell just the pulleys, which are the part that usually fails first.
Unless you have unusually high miles, this car is a little 'young' for a tensioner or pulley to be worn out. Has anyone worked on the car who may have left something loose or missing?