Remember having a Big Wheel as a kid in the 60s and 70s? Remember how the back wheel would tick because it had some kind of plastic piece rubbing on the plastic spokes to make the noise? It sounds like that.
I cover this in my book. The noise is caused normally only in the number 2 position of the fan setting, and is a function of the way that the exotic Alfa-Romeo sourced HVAC control regulates the fan speed.
The sound is a sort of "clicking" sound.
There is no known cure other than switching to a different fan speed.