I answered that question with my post. =]

If you move your arm fast enough past your ear, you'll hear a sound. So it's right in that stretching your arm into the air would cause (not "be", if we're being pedantic) a sound, just a very small one that most people wouldn't hear. In fact, some sounds aren't heard by people at all. But yes, a sound all the same. That's why there are dog whistles. We can't hear them, but the vibrations exist, and it turns out dogs can hear them. It would be silly to dismiss something as soundless just because we don't hear it.