The noise you are hearing is not a growl, right? If it is, it may be a bad A/C Idler Pulley Bearing. Here is a TSB:
But, to answer your question on how to adjust the A/C belt tension:
(Use the image in the TSB as a guide, so you know which is the pulley center nut, and which is the adjusting bolt.)
1. Loosen the center nut on the pulley (Loosen, don't remove).
2. Turn the adjusting bolt to get proper tension.
3. Re-tighten the center nut on the pulley.