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I think what youre hearing is the "efficient dynamics". Behind your front kidney grill are two vents that open and close depending if the radiator needs cooling or not. Its supposed to reduce fuel consumption when they are closed. This is from BMW's website
Open to increased efficiency.
A constant airflow through the radiator increases air resistance and affects aerodynamics. Electrically controlled vents in front of the radiator open and close the cooling air inlets so that the vents are only opened if cooling air is required. When the vents are closed, air resistance is reduced, as is fuel consumption
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