Yes, sometimes they sing: Some propellers in service produce a high-pitched noise, often referred to as “singing.” This sound typically is a clear harmonic tone much like a humming or a ringing wine glass. More of an annoyance than anything harmful, the causes of singing are not completely understood. Many theories have been put forward to account for the phenomenon of singing, but it appears to be affected by critical factors for which the theories make no allowance. For instance…