This is a design I came up with as a challenge for myself, to build a working version of a "siren whistle" like those sold in novelty shops or as party favors. Obviously a little bigger, and incredibly over-engineered (designed for 2mm steel shaft and 2x 2*5*2.5mm ball bearings) but I have a fair amount of confidence that it will work. May be updated from time to time as I tweak it. I'm not convinced I will have the lung capacity to actually use it, really. Might have to be compressed-air powered.