WARNING: This page is about a different car, the 2003 GMC Safari and 2003 Chevrolet Astro. However, it is still accessible from the selected car via links, so may be relevant.
Fig 1: Noise Diagnosis Chart - Blower Motor (Steps 1-7)
Courtesy of GENERAL MOTORS CORP.
Fig 2: Noise Diagnosis Chart - Blower Motor (Steps 8-10)