I Can't Hear Anything From Here, But There's A Chance That A leaf Or Other Debris Has Literally Hit The Fan And Stayed There.
Stuff sometimes gets through the air intake in the cowl ahead of the windshield.
Have you tried differnt fan speeds while selecting different places for the air to exit (defrost, floor, dashboard vents) ? Sometimes this will dislodge it.
A mechanic / technician should be able to give a listen and verify this. Some cars give easy access to the blower assembly and others do not.
Without hearing it in the vehicle this is only one guess at a possible source of the noise.
I suppose it could be a vacuum line that is leaking, instead.