03 Aurora 4.0 - whistling noise when revving

When I rev my 2003 Aurora, I have been getting a whistling noise. At first I thougth it was a belt noise - as I am driving. But the other day I kept the car in park and simply started revving it to see if I would hear the same noise. As soon as I rev itto approx. 2000 rpm, I I hear that same whistling noise. It didn't sound like a belt noise to me then. Can anybody help me get rid of this noise with some detailed steps? The noise is almost constant, but it almost seems like the whistling becomes less after a longer drive.
Thanks

idler pulley

Not saying this is it especially since I can't hear your noise, but when my 02 made a type of whistle - wuing type sound it was the transmission.
Have you ever had the transmission fluid changed or serviced?

If not drop the pan and the contents inside will tell you allot.

I just found out today that the whistling noise is actually coming out through the exhaust! It is a high pitched whistle when staying behind the car. Whether the car is parked or going, you can hear a distinct whistling noise. It doesn't matter whether the motor is cold or warmed up either. Even though I now know the source of the whistling, I have no idea what is causing this!! Has anybody experienced this or have any ideas on how to go about eliminating this noise!
Thanks