It seems to be a combination of multiple sounds. I've tried searching for the file but cannot seem to find it. I know one of the sounds below is included with the Speedbarge sound.
Code:
Sound\Spells\FX_Bubbles_Large_Loop_01.ogg
Sound\Spells\FX_Bubbles_Large_Loop_02.ogg
Sound\Spells\FX_Bubbles_Large_Loop_03.ogg
Sound\Spells\FX_Bubbles_Medium_Loop_01.ogg
Sound\Spells\FX_Bubbles_Medium_Loop_02.ogg
Sound\Spells\FX_Bubbles_Medium_Loop_03.ogg
Sound\Spells\FX_Bubbles_Small_Loop_01.ogg
Sound\Spells\FX_Bubbles_Small_Loop_02.ogg
Sound\Spells\FX_Bubbles_Small_Loop_03.ogg
Here is a good list of all the sound files:
http://www.isoverninethousand.info/p...w/raw/16691792