Best whistle on thingiverse.. The first couple I had some air leaking from the sides due to under extrusion.. After that was fixed.. Man o man.. These are loud.. My kids all have them on their backpacks for emergencies.. Have them on keychain.. I have them everywhere!
Note: if you're having trouble and feel like you need to blow hard.. Check the sides.. They are probably leaking air. These should be extremely easy to blow to make whistle.. You'll know when you have it right!