If the noise is not dependent on the car moving, and only happens when it is cold I would turn on an accessory like the lights or AC in the morning as soon as you start. If the noise is worse, prolonged or even louder it is most likely an idler puller on the belt drive system. I had a similar problem and let it go until it became easier to trace. You can use a mechanics stethoscope and place it on the bracket of the idler pulley close to the pulley itself and listen for excessive bearing noise. They are not expensive and very easy to replace.