Before you do anything, try to figure out what exactly you heard. Were they actual words being spoken, or simply sounds like that of an animal? Can you relate this sound to any sort of experience you had that made an impression?
Once you are able to diagnose what you're dealing with, do proper procedure. Banish all negativity from the room, cleanse and purify any objects of often use, and set up a four-corner barrier. Keep the barrier up and do not go into any meditative state for about five days. After five days, cleanse the room and yourself, and remove the barrier. If you are attacked again, I would suggest strong visualization of anything that can protect you in AP. (Shield, brick wall, bubble of protection, ect)
(Note: The barrier is done in the room to allow the positive energies to heal and manifest)