I've got the same issue as well. I think it might have started on mine shortly after installing/uninstalling a sound file editor app to crop ringtones etc but can't remember for sure.
Think it makes the sound after playing a specific type of audio file. Mine does not do It after .mp3, .amr, and some others... seems to only be after .wav files. I also notice it in some games which I presume have .wav sound files in them.
Any ideas at all would be much appreciated.
