As of World of Warcraft v1.7, a new feature was implemented where a dong, or chime, would go off every minute or so while you were queued for a battleground. This proved to be annoying, and has yet to be removed.

The sound that plays is called PVPEnterQueue.wav, and is found in the patch.mpq file, in the directory Data/Sound/Spells. I found this out by using MyWarcraftStudio.

I have created a tempory fix for this annoying queue sound. The following ZIP file contains a silent WAV file named PVPEnterQueue.wav.

Extract the contents of the ZIP file to your WoW installation directory, and it should create the Data/Sound/Spells folders for you, and put the silent PVPEnterQueue.wav sound in the Spells directory.

As this is a blank sound, no sound will be played every time the game accesses this file. I'm pretty sure this sound file is only used for the BG queue, so it shouldn't affect any other element of the game.