Imagine taking the cross section of a standard referee's whistle and the revolving it about an axis to form a toroidal shaped sound cavity - that's what Robert Levavasseur did when he patented this whistle in 1956. Word has it that in testing a giant 6ft version of this type of whistle, the sound was so loud that it ruptured the internal tissues of Levavasseur and two of his colleagues.
This whistle is a little simpler (and smaller) than the version in Levavasseur's patent (http://www.google.com/patents/US2755767). While this version does whistle, I'm sure a lot could be done to optimize the sound. In any case, it will certainly grab the players' attention out on the field!
Update: I've added a second version (whistle_1.stl) which seems to whistle a little more reliably. It has built-in supports designed specifically fro 0.1mm layer thickness.
You can probably print this whistle with 0.2mm layers, but the smoother the internal surface, the better it will whistle. Note that the built-in supports are designed specifically for printing with 0.1mm layers - using 0.2mm layers may make them difficult to remove.
I've included two STL file versions - one with a break-away support structure built-in and one without that will require you to turn on supports. The one with supports also has bridging ribs on the inside of the torus that don't need to be broken away. I recommend using the version with built-in supports because the slicer generated supports will be devilishly hard to remove and won't leave as smooth a finish. The STL file without supports, though, makes it easier to see how the whistle is constructed.
Whistles are really hard to make whistle properly. I've designed simpler whistles, printed the same design twice and had one copy whistle great while the other copy just wheezed. I went through a lot of iterations to get this goofy version to whistle. A tenth of a millimeter here or there will significantly alter the sound, and surface smoothness is very important. I've included the original Solidworks file and it'd be great to see other people improve upon this design.