It could be a piece of foam stuck in the 'hamster wheel' blower fan. The fan is located on the passenger side of the dash. It's not very difficult to check, it takes maybe 20-30 minutes, and most of it is reinstalling one plastic piece.
Here is the DIY I used, unfortunately some of the pictures are gone:
https://sites.google.com/site/mikefocke2/hvacblowermotor