My 2 ton central A/C at home has frozen up again. It froze up after freon was added.?

The unit had been working fine but Sunday it was around 95 during the day and the unit froze up around all the pipes and compressor during the night. I shut it off to defrost and a friend who works at Traine added about 1LB of freon. He figured it probably had a slow leak since the unit had had nothing added in years. It seemed to work fine and the house cooled from 83 to 75 but someime during the night it has frozen up exactly again. I shut it off and turned on the fan. What does it sound like is happening?