I believe that one is The Monsters Without. It is about 8 seconds long and can be found in the original game. The Monsters Within also has a similar tune. These songs can be found in:
Program Files/Left 4 Dead/left4dead/sound/music
From there, you can find Monsters Without in the "safe" folder and Monsters Within in the "unalive" folder.
Left 4 Dead 2
Left 4 Dead 2
