All power adapters or transformers (the brick) will make a humming or buzzing sound to some degree. However, if this sound is excessively loud, then the issue could be caused by either a problem with the transformer or the quality of the AC line voltage.
NOTE: If the noise intensity varies at different times throughout the day, then the issue is most likely due to the quality of the AC supply voltage in your area. Otherwise, the transformer may be faulty. If it is the case that the transformer is faulty, service may be required.