It sounds like you have a faulty A/C blower motor or there could be some debris in the motor fan causing the abnormal sound and poor air flow from the A/C in your 1999 Buick Regal. In any case the blower motor will most likely need to come out for inspection.
RepairPal Certified shops can diagnose and repair your 1999 Buick Regal. They provide transparent pricing with a minimum 12-month or 12,000-mile warranty on repairs and services.