While on a road trip to Arizona, I listened to The Lord of the Rings movie via a DVD player plugged into my car stereo. From only being able to listen to the audio while driving on an endless stretch of road, I automatically began to pick apart the different sounds. By not seeing the creature making the harsh noises, I better deciphered what was used to create their "voices" - The Nazgul for example, are mostly made up of pig squeals.
Here are some of my creatures:
Crank it up! The second sample has much more low end than the first. But, the first does have a cello layered behind it.