1999 GMC Suburban loud noise

I recently bought a 1999 GMC Suburban. After the car has been driving for awhile there is a loud whirrling/ whistling sound that comes from the front of the car. It getting so bad that you can hear the truck coming, its very embarassing. My husband who knows absolutely nothing about cars says that it's the fan belt. Another gentlesaid that it is the timing belt. Do you have any idea what it could be and is it very expesnsive to fix??