Pulleys

I need some assistance....
I have a 97" Pontiac Grand Prix GT and I think the pulley right beside the alternator is going bad. I hear the alternator running, but the pulley right beside the alternator is making noise. Say if you was standing on the passenger side of the car and you was looking at the alternator; it is located on the left side of the alternator of the car. I do not know the name of the pulley but I need to know because I need to replace it so could somebody help me with this please.

Not sure if its the tensioner pulley or the idler pulley, but you could take it off and have a parts store match the size, or ask to see both the idler and tens. pulley and see what looks like the one on the car

Thanks, I will do that