The outbursts began in 1984 and lasted for about four years, until NOC hit sexual maturity. NOC likely learned to imitate humans by listening to them speak underwater and on the surface.
The whale's human-like calls are several octaves lower than normal whale calls, a similar pitch to human speech. Researchers trained NOC to 'speak' on command, and determined that he makes the sounds by increasing the pressure of the air that courses through his naval cavities. They think that he then modified the sounds by manipulating the shape of his phonic lips, small vibrating structures that sit above each nasal cavity.
A recording of NOC's speech is embedded in the story. He sounds remarkably like a kazoo."
Link to Original Source