I wanted a decent small loud whistle I could print and give out to people where I work. I downloaded several whistles and printed them but could not find one that worked reliably. So I decided to make my own. I started with a small metal whistle that I have carried for years, and tweaked the design until I got something that is loud and, at least on my Rostock Max V2, prints reliably.
One of my goals was a reasonably fast print. At first I had to print at 0.1 mm to get a good whistle, but I found by slowing the print speed down just a little (35 mm/sec) I could print this at 0.2 mm and get a good working whistle, with a print time of around 35 minutes. I mostly use PLA but if you get strings you have to clean them out or the whistle won't work well. I found that a medium size nylon cable tie makes a good tool for cleaning out the fipple (that's the name for the slot in the whistle that channels the air into the notch - fun word, eh?).
The internal tubes are 41 and 40 mm in length; this determines the tones and the difference sets the whistle's beat or warble.
My son Byron added the name "Super Sonic".
I hope this prints for you as well as it does for me, and that you find it useful. Remember, if you are heading into the back country, please carry a whistle!
Update 06/08/16: Added a new version. Thingiverse member Qrome designed a nearly identical whistle, and I liked the wedge shaped fipple in his whistle. I wanted to see if I could figure out how to do that in FreeCAD. I discovered a cool trick for doing this, message me if you're interested in how it's done. This version is VERY LOUD!!!
I find this prints best at a somewhat lower speed - I used 35 mm/s.
For PLA be sure to tune your printer for minimal stringing.