Seems like there is a small hole somewhere in the line that lets moisture in.
If the system had even the smallest hole, the pressure inside would deplete all your freon and there would not be any cooling period. But Nick is right,
I'm not sure how, but moisture sometimes accumulates inside these closed systems and when subject to freezing temperatures will turn to little drops of ice.
Take the unit out and have a service technician remove the freon and either strain it or replace it.