My 03 expedition used to make that noise when the rear AC is turned on. For some reason is dissapeared by itself.
It sounded like something is obstructing the fan bcos, the sound gets more frequent when we increase the fan speed.
There is a removable panel/cover above the rear ac vent. You could remove that and see if something is stuck in there.
Good luck !
Tuesday, December 11th, 2007 AT 11:48 AM