It's either your belt tensioner/idler pulley or both. You should replace both at the same time. Check the DIY's on how to do this.
Current Vehicles in my stable:
2005 BMW 330i - 126K miles
2005 Mazda Tribute s - 117K miles
1998 Ford Taurus SE - 115K miles
1987 Ford Mustang LX 5.0 - 248K miles