Call of Duty: 1914 is a total conversion of CoD2 set in World War 1. Delve into the first modern conflict with the trenches of the First World War. Play as the forces of the British Commonwealth, French Republic, Russian Empire and the American Expeditionary Force against the forces of the German Empire, Austria-Hungary and the Ottoman Empire.
This is a Whistle sound, famous stuff from WW1 used by officers. More versions are possible in the future.