Could be the water pump bearing going out. Take the serpentine belt off and run it for a few seconds. If the sound goes away, it's either the alternator, water pump, power steering pump, a/c or belt tensioner/idler pulley. Sorry I can't be much more help than that.
Cory is right on w/what he said. This is a quick way to narrow it down to where the noise is or isn't coming from....joe
