Air Pump Suction Hose O-Ring

A worn o-ring on the intake side of the air pump suction hose will result in a minor air leak, which allows unfiltered air into the intake tract. While this will not cause a check engine light, it can be one of many causes of an audible whistle while cruising at part throttle. 

Not available by itself from BMW, this o-ring typically would need to be purchased as part of a $200 air pump hose assembly (11727838504) and is found where it connects to the intake boot (11617838286).