Do you get the sound while your car is at high rpm?
It could be your water pump, or some of the rubber thingies which go from the engine to the alternator? I dont remember the English word.. sorry..

Theese are just my guesses.. Check if the rubber thingie is loose.. if it is, you probobly need to swap it. This would make sound if its worn out, and engine is cold. It would gain more traction when it's reached a certain temp I would guess.