Only accelerating surfaces, not steadily moving ones, generate airborne sound.

Evidence

The key to sound production by a surface is acceleration. A surface moving at constant (subsonic) velocity doesn’t produce sound: no cause for pressure pulses to be emitted exists. The air in front of the surface is traveling with it already. But if that surface (or part of it) suddenly changes its velocity—that is, a large acceleration—the adjacent molecules are “surprised” to see the surface suddenly moving away or coming toward them, causing a rarefaction in the former case and a compression in the latter. — Eric J. Heller, Why You Hear What You Hear- An…, 645

Categorization

• DEFINE: The load-bearing mechanism: steady motion carries air along with it, only sudden velocity change creates pressure pulses.