While blend door motors do have problems, from my experience with so many of all the GM models from 1999-2006 with HVAC issue, the main problem I found was the tiny female terminals that are supposed to mate and grab the small square pins on the blend door motors would typically become loose over the years. I have had to remove the female terminals out of the connector head shells and pinch them closed enough to properly grab the male terminals on the blend door motors. I have fixed countless HVAC blend problems doing this without replacing blend door motors and have fixed many vehicles that had blend door motors replaced and still had problems.
While you have the individual female terminals out of the head shell you can put them on the male terminals and feel how sloppy and loose they are and once you pinch them closed enough you can test them on the male pins and make sure they grab correctly.
At least on the 1999-2006 era blend door motors, when either the blend door motor goes bad or the connections are bad there is usually a constant "knocking" noise as the blend door motor attempts to move the expected spot.
