New belt but MUCH Noise!

Hey i got a Cavalier 98 LS Automatic (Without A/C). I just replaced my serpentine belt because the old one was damaged. I bought a new one in a Canadian Tire for 30$ (On the box the name of the Cie is Torque). So i installed it start the engine all was fine. But when the engine start to be hot (Like 70-80 Celcius) the belt make a very BIG NOISE and then the light start to loose power. At idle i got a small noise from the belt.. but when i accelerate *2000 rpm* its terrible.. But at 2600 2700 RPM Bang no more noise! So is it the belt is too long or the belt tensioner is too old?

is your belt tensioner working properly? or does it not work? that can do it

Update.. the belt tensioner is good .. the new belt was too long so i put the old one... Where can i buy the same belt?