Odd sound/signal

Hi everyone,

This could be kind of odd, but I'm hearing a sort of signal/sound when I listen to music while on earphones. It is like an indication for something and similar to sound when e-mail arrives. It doesn't disturb me, but could be cool to know the reason for this signal.

Thanks for any of your help.

P.S. I have just purchased MBAir so I'm not familair with full functionality and stuff like that:)

Best regards,

Dmitrij