He Who Saves His Country Does Not...
Society & Culture4 mins ago
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For more on marking an answer as the "Best Answer", please visit our FAQ.It's the difference in the pressure of the air as it flows around objects. The differing pressure is often caused by a venturi effect... when air is compressed, as in passing through a crack in the wall or even flowing over a branch and then expands as it completes the passage, which causes the different pressures which causes vibrations detected by your ears. In some cases, the object over which the air is passing can bivrate causing sounds as well. The gas law at work here is Bernoulli's Principle, as well as others...
I think that the whistling sound probably comes from a phenomenon called vortex shedding, which occurs on alternate sides of an object in wind.
This can be very destructive when if coincides with the natural frequency of the object e.g. the Tacoma Narrows Bridge. This is why tall thin chimneys often have spirals going up them, so that the vortex shedding is disrupted.
For a small object, the vortex shedding could occur at a frequency that we would hear as whistling.
http://www.princeton.edu/~asmits/Bicycle_web/separation. htmlJust after figure 13 there is a bit of explanation about this topic.