I have had this noise bothering me for a while...

Sounded like something was rattling around behind the glove box.
Its the fan motor, it is mounted vertically and the armature has the ability to move up and down when you go over bumps... Clunk Clunk Clunk.

take the fan assembly out and remove the impeller put a few orings or a nylon spacer in there so that it cannot move... dont make it too tight or you will cause a bind in the shaft.

Noise fixed,

Much Better...

 

Good info mine is doing that too, I could not figure out where it was coming from.