Clean as a Whistle

Why do we say "clean as a whistle"? Is there some inherently clean property about whistles? When I think of whistles, I think of a little metal thing that a person blows into, possibly with bad breath, spit, boogers, you name it, flying into it. That's anything but clean in my book. Kind of gross actually. Next time someone says clean as a whistle, I think I might bring this up:

as clean as a whistle

without any dirt or marks, smooth and clean

The hen squeezed, and out popped an egg as clean as a whistle.

When I think of clean as a whistle, it just doesn't get any better than a chicken pushing a shit-coated egg out of its ass.