if you have never changed ancilary belt, it's time

basically the belt is loose.  if you know you have changed the belt in last 3-4 years, you can adjust its tension - there is a long bolt you can adjust.  if you aquired this car not long ago and you are not sure, turn the belt back (the side contacting pulley) to look for cracks; if there is any sign of cracking, change it.  the belt is cheap about $10, diyable; shops may charge $30-40 for labor...