DIAGNOSIS This is the sound of condensate water moving within the internal drain pan or being pumped by the evaporative condenser. It is a normal operational sound for this model.
FIX No fix is required. The sound is most noticeable in high-humidity conditions when the unit is removing more water from the air. The design uses this collected water to help cool the condenser, improving efficiency.
IF THAT DIDN'T WORK If the sound is accompanied by water leaking from the unit onto your floor, the internal drain system is blocked. You must unplug the unit and tilt it carefully to drain any standing water from the pan. Check the internal drain hose for kinks if you are comfortable with disassembly.
HEADS UP Do not manually drain the unit via the rubber plug unless it is in storage mode or leaking. The self-evaporation system requires that water remain in the reservoir during normal operation.