I have been having this happen more often lately. Seems to happen most of the time when there is a lot of stuff going on… occurs frequently in the Claw of Jormag or WvW.
It sounds like the emergency broadcast system sort of… a pulsing sound. I am using a plantronics USB headset. I have tried alt tabbing out of game, but it actually persists even when the game is not in focus. I have also tried switching the in game sound from the headset to main speakers and back, but the sound persists until the client is restarted. Also lately is has been freezing the GW2 game when it has happened.