1996 Buick Century From Alternator belt

Noises problem
1996 Buick Century 6 cyl Front Wheel Drive Automatic @57K miles

I recently recieved this car as a gift from my wife's grandmother. It was rarely used reason for low mileage for that year. My question is recently I noticed a sound coming from the main belt that drives all the accessories and noticed that the lower wheel seems to be spinning out of sequence ( not all the time or as fast as the others ) I have no clue? How to fix. Thank you Steve