In the course of propulsion experiments in the ring-channel and rotating-boom facility at the Aerojet Engineering Corporation, it was discovered that certain struts which are used to support underwater bodies would “sing” at surprisingly low speeds, e.g., 5 to 7 knots. Tail surfaces also sang. The note emitted was a clear musical tone in most instances, and, as the speed was increased, the note would fade, disappear, and then reappear as a note of higher frequency. This singing caused considerable apprehension as it was feared that the strut would fail at high speeds due to build-up of the vibrations. Analysis for flutter stability and torsional divergence had been conducted, and it was known that the operation was well away from the ranges where flutter or torsional divergence might occur. However, the severity of the singing did not tend to become greater at high speeds, and there were usually quiet speed zones at high speeds as well as at low speeds.