The cabin fan, when the recirculating function is enabled, seems to be put under a heavier load, so maybe the bearing is a bit worn?
It should make the same noise if you select "recirculate" function.
One other thing, maybe scan the car and look at the recirc flap motor when selecting recirc as that motor might not be moving the flap(s) correctly and strangling the fans air flow? I think that the recirc flap motor moves two flaps and just the one as its description suggests.