Hi Rich, Think violin, or bass fiddle. They all have strings with different tensions on them. If you blow across them with an increasing wind speed, at some point they will become excited - and "sing". Shrouds / forestay have the same effect. On my big boat I knew that the winds were between 20 and 25 knots when my rigging started to sing. A natural phenomenon. When it starts singing is a question of the tension in the fore stay / shrouds. Haven't calibrated my M15 that way yet so can't add any definitive words of wisdom, but rigging singing is natural. Connie