Happens to me too, usually when there’s a lot of stuff going on i.e. in a WvWvW siege or things like that.
I had bass boost enabled in my sound card settings, and when I disabled it, the “woowoowoo” stopped but I was also left with no sound at all in game. It returned when i tried changing the audio quality in game and now I can’t get rid of it.
Hope this gets fixed soon.