It is quite possible that there is a leaf in the blower motor... because of the fan design it sounds almost exactly like a card in spokes. Unfortunately the vent area in front of the windshield isn't sealed all that great so leaves can land on your car and make their way into the motor.
The blower motor is in the passenger footwell. You take off the panel in the footwell, then you can take out the motor. It is held in by some screws, I don't recall the size. It is fairly simple. Then empty out whatever junk is in there. I did this early in the year which was probably the first time it was done on this 9 year old car... there was a fair amount of leaf debris in the bottom.
However, if it is just one leaf, you can also try to just chop it up with the fan... just watch out cause pieces will fly out the vent at you