When you say leak, you mean it is dripping out of the bottom of the unit?
You could have condensation building up because the return air and discharge are mixing. If humidity is very high this will be a problem. Remove the inside cover and you will see a liner between the return and discharge vents. Look at it and see if there are any holes in it. You can fix it easily with duct tape.
You could have a loose roof seal that gets overwhelmed when it rains combined with all the discharge moisture.
I would pull the inside cover, check the liner, and also tighten the four bolts a turn. See if that works.