50Hz hum from macbook air headphone port
Most of the Macintoshes I've had over the years have had a buzz in the headphone port. It is especially noticeable with low impedance headphones. Some have even had a click when the sound is turned on (i.e. when an alert sound plays, or a song is started in iTunes).

You could try taking it to the Genius bar, they might replace it if it is still under warranty. There could be a short somewhere, or it could just be considered "normal" which is why audiophiles use digital output.