I had this same thing on my 2002. I took apart the left rear cargo area
to get at it. The blend door was not broken, but the actuator that opens
and closes the door was going crazy and that's what was making the noise.
I discovered I could get heat or cold, but that the actuator wouldn't let
the door stay in one position. So when it got to cold, I unplugged the
connector and there it stays. I seldom need heat back there, and when I
do, there are still the vents in the back bottom of the center console. I
think I remember reading about this and there may be a real fix that
involves rewiring the circuit, but for me, this hasn't been needed.
However, this may be covered by your extended warranty, especially if its
a factory issued one. Ask  your dealer. I would.