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partial acceleration

  1. X5 E53 (1999 - 2006)
    I have a 2005 X5 with approx 35k miles. I noticed there is a whistle that occurs when I accelerate slowly at lower speeds. If I punch it, it is not there. I saw another thread relating to a 3 series sedan with the same issue that said that it was the crank case vent valve. Any thoughts?