".As every old-timer can tell you, a good whistle made from a reed or a piece of wood emits a clear tone - but it is easily damaged. Even small particles of debris, or a few drops of moisture will change the sound of a handmade instrument. In order to emit the pure notes intended by its maker, a whistle has to be absolutely clean. Anything or anyone as clean as a brand-new whistle or as clear as its sound is bound to be good. All of which means that an organization or person called as 'clean as a whistle' has been judged to be guiltless or flawless." (From Why You Say It: The Fascinating Stories Behind over 600 Everyday Words and Phrasesby Webb Garrison, Rutledge Hill Press, Nashville, Tenn., 1992.)