Have you ever blown air across the opening of a bottle and heard a tone?
Something similar is happening in your window. Your window must have a small opening somewhere and as the wind blows across it, it makes a tone. You might notice that as the wind speed changes, so will the sound.
Lower wind speeds will produce quieter sounds while faster wind will produce louder sounds. There is a point where very, very low wind speeds will produce no sound. The wind has to blow at the right speeds in order for an effect known as resonance to occur.
The size of the crack in the window will also change the sound. Very small cracks will sound higher than very large cracks just as big bottles produce a big, low sound and little bottles produce a small, high sound.