The common causes of such noises are the power steering belt, the pump, or low fluid. Does the sound come from the front of the car or from the inside? If you can't tell ask someone to stand by the front drivers side of the car and listen. My steering column sometimes moans in strange ways when I turn. I haven't bother to figure out why because I've grown fond of the moaning sound and it never does it when I try to find the source.
The fluid level is easy to check, it is the little black lid about 4-5" in diameter with a wing nut. It's mounted on the driver's side fender wall just behind the battery. When you remove it you should see a healthy amount of fluid in it. Ideally the fluid should be slightly red in color but it becomes brownish with age. Good luck.
