1) a clogged drain tube that is supposed to drain the condensation collected by your AC evaporator under the dash. If the catch tray fills up without being drained, it will slosh around as you drive, or just overfill as you continue to use the AC. After using the AC for the day, you should see water dripping down to the ground at about the windshield area. If you never see this dripping with the AC running, then that might be the problem
2. You have a clogged drain in the fresh air cavity under your windshield wipers, where rain can collect and form a little fishtank in there.