I believe it's just a noisy P/S pump. just about every late model I know of is loud like that. As far as I know, it's no big deal. Like said above, just make sure your fluid is ok. Do you have any other problems with the P/S system other than the noise?
But as far as I know, it's just one of those things.
if u have the car on and jus leave it in park and turn the while not moving and it make the noise and the noise is constant then its power steering check fluid if its full then ull be fine and ur pump is just noisy