Audio of ambient sounds "pop" for a fraction of a second at full volume second. This seems to happen across all maps, and most often with the "rushing water" sound.
I've recorded a video and uploaded it to youtube for your viewing pleasure, and so it's a little easier to understand what I'm talking about. =)
Skip to 30 seconds.
http://www.youtube.com/watch?v=pfMsx4rFO7o