timing belt noise

i bought this car a few months ago and its been running fine but now when i let the engine idle i hear i whistling noise.it sounds like its coming from the timing belt.if i rev the engine it disappears for about 20 seconds and comes back. but i dont hear it when im driving, only when i come to a stop.not completely sure if it is the timing belt but its in that area. anybody have a clue wat it is

When water pump bearings begin to go bad, there can be a whining noise...but you would know if the pump was bad because your temperature would rise above halfway. How many miles are on the car, and do you know if it has had the timing belt replaced and when? If you're up to date on your scheduled maintenance, and your car isn't performing poorly or gauges showing problems, you may be over-analyzing the noise ;). Take it to a mechanic to let him troubleshoot it if you are still unconvinced.