Car makes screeching noise when turned on with AC on

Possibly ac clutch going bad. But more than likely its either a belt tension issue, bad/damaged belt or idler pully issue. Use a small piece of PVC pipe or hard vacuum cleaner hose to locate where sound is coming from. Raise hood and crank up car. Carefully position pipe in and around belts and pullies while holding other end to your ear. The sound will be magnified thru the pipe allowing you to locate the problem. When you locate the problem replace effected parts. Use extreme caution not to get it hung in belts or pullies while doing this procedure.