the way you describe fitting the prop had me thinking.'the nut fits right in and tightens down by spinning it'???????
i presume it is a gws style prop with hexagonal mouldings each side,if so i may have figured out the screeching sound.there should be a nut on the motor/gearbox shaft,the prop engages with this nut,you then place a washer onto the shaft and use another nut to secure the prop.is this what you are doing?