2002 Cadillac Deville whistleing sound when I start car

Lately when I start my car I hear a whistling sound for about 20-30 seconds then it goes away. I notice if I turn the car off and turn it back on within 30 minutes or so it won't do the sound, but when car is park for more then 1 hour it will do it again when I start the car. Lately I notice that sometimes it will do it when im driving but only last about 15 seconds then it will go away. My friend said it sounds like its something in the transmission. Any idea of what it may be? Thanks