Alternator?

I have an 08 P71. I recently started getting a noise that is intermittent that comes from what I think is the alternator. It's strange because it will happen after the car has been run for a little while, and never seems to happen when freshly started, and when it's not "squeeling/grinding" it's absolutely normal sounding. I don't get any lights on the dash and no symptoms other than the noise. Once it starts, It keeps going even when the car is at a stop. The sound does not change with the throttle, it just comes and goes, and is Very noisy when listening from under the hood. I just want to know if I should go ahead and replace it, or if there is something else that could make a noise like that.
-Much appreciated

rockauto.com has several to choose from.

First, you really need to determine if it is the alternator. Use something like the center cardboard tube of a paper roll up against your ear, and when the car is idling AND make the sound , LISTEN! Try to pinpoint the noise. If it is the alternator, replacing it isn't that hard. Like the previous response, go to www.rockauto.com for a replacement that is cheaper than Auto Zone or Advanced Auto Parts. Also, you may want to go to www.crownvic.net to check out other possible problems.