This sound is from Scout Sounds: Team Fortress 2

Team Fortress 2 is a 2007 multiplayer first-person shooter game developed and published by Valve Corporation. It is the sequel to the 1996 Team Fortress mod for Quake and its 1999 remake, Team Fortress Classic.
See also: Team Fortress Classic, Scout (name), The Scout, Scout, Fortress of Solitude, Saxxy Awards.