"Clean as a whistle"

CLEAN AS A WHISTLE - "One possibility is that the old simile describes the whistling sound of a sword as it swishes through the air to decapitate someone, and an early 19th century quotation does suggest this connection: 'A first rate shot.(his) head taken off as clean as a whistle.' The expression is proverbial, at least since the 18th century, when Robert Burns used a variation on it. More likely the basic idea suggests the clear, pure sound a whistle makes, or the slippery smooth surface of a willow stick debarked to make a whistle. But there is also a chance that the phrase may have originally been 'as clean as a whittle,' referring to a piece of smooth wood after it is whittled.'" (From the "Encyclopedia of Word and Phrase Origins" by Robert Hendrickson (Facts on File, New York, 1997.)

: Another source states: "Robert Burns, in his poem, 'Earnest Cry,' used 'toom' ('empty') rather than 'clean' - 'Paint Scotland greetan owre her thrissle; Her mutchkin stoup as toom's a whissle' - and other writers have had the whistle clear, dry, pure or other adjective. The basic intent, however, is to indicate that, for a sweet, pure sound from a whistle or reed, the tube must be clean and dry." (From "Heavens to Betsy & Other Curious Sayings" by Charles Earle Funk, Harper & Row, New York, 1955.)

: And a third: ".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" by Webb Garrison, Rutledge Hill Press, Nashville, Tenn., 1992.)

Alternatively the origin may be the clean appearance of a just carved wooden whistle. Personally, I think it may well relate to locomotives where the brass, especially the whistle was always bright and gleaming.

I found the preceding info here.