Humming noice under hood

i have this humming noice under the hood when I sit at a stop light and dont know what it is the car is up to temp but It just makes a noice and it dosent sound good any help would be great. thanks

I have a 97 cavalier. 5-speed, I would get a loud ringing noise when the car is idling. It happens when i started the car up and after i drove it. no one really can find out what is it. they told me to just leave it alone.

If its a five speed car it could be the through-out bearing. Its nothing serious for it to hum, but if it growls or sqeaks, thats bad.

Open the hood and locate the source of the hum. Pay more attention to the alternator and waterpump. If it is undecisive, take the belt off (remember the routing) and spin the pulleys by hand. The water pump should not have any play at the pulley, if it does, replace it. If you spin it fast, you will hear a scraping noise.
Good Luck.