Noise cancelling headphones work by identifying the frequencies of noise outside the headphones and creating the exact opposite frequencies. The two sets of frequencies cancel each other out so the brain thinks that no noise is present.
As this happens there will always be a very low "white noise" effect. This low level "white noise" is considered to be silent because most people are used to having back ground noise present in everyday life so often don't even realize that this "white noise" is present. If your hearing is very attuned you may be more aware of this "white noise" however if audio is present in the headphones then the "white noise" is completely covered over and usually not detected by the human ear.
Please check out this video (http://www.youtube.com/watch?v=1EwkaRvIduw) which will show some of our Noise Cancelling models in action. The model shown may be different but the concept is the same.
NOTE: Noise cancelling is most effective for repetitive low level frequencies (like engine noise) but will not block higher level frequencies (like people talking) as much as these frequencies are needed to allow sound to work in the headphones.