As Gally noted, Kenny is actually talking under the hood. This leads to the next question: what is he singing in the credits song?

A fiend of mine managed to turn the audio of the opening credits of an episode from the first series into a wav file. Then he used a sound editor to slow it down without changing pitch. After repeated listenings, he made out the following words which are recognisable once you know what to listen for:

Kenny says I like girls with big vaginas, I like girls with big ol' titties.

Update: break says actually, Parker & Stone already revealed the intro song Kenny lyrics in interviews ages ago.. but what your friend got is almost 100% correct.
Update 2: ConfusionTheWaitress says I heard that in seasons three and four Kenny's line in the theme song is changed to 'I have got a ten inch penis' x2