Diagnosis
Restricted airflow. The evaporator coil behind the filter is either frozen over or clogged with dust.
How to Fix It
Unplug the unit. Remove the filter and look through the grille at the aluminum fins. If you see frost or ice, let it thaw completely for 24 hours. If you see dense dust, use a vacuum crevice tool to gently clean the fins.
If That Didn't Work
If the coil is clean and frost-free, the fan motor might be failing or the fan blade has come loose on the shaft. This requires opening the top cover to inspect.
Heads Up
Low airflow will cause the coil to freeze solid. If you see ice, the fix is to thaw it AND find the root cause: dirty filter, low room temp, or low refrigerant.