I changed the auxilary belt at the weekend as it had snapped, all seemed ok untill this morning when i got in the car and had a funny squealing noise which i can only asume its the tensioner, can this part be changed?

I have also noticed after driving and becoming stationary it idles a little higher than usual for a couple of seconds, then drop, any ideas whats causing this as it didnt do it priior to changing the belt.