DIAGNOSIS: That's normal refrigerant flow noise. The refrigerant moves through the coils and settles when the compressor cycles off.
FIX: No fix needed—it's a characteristic sound of this and many portable ACs. To potentially minimize it, ensure the unit is sitting perfectly level on the floor. A slight tilt can make the refrigerant drainage within the coils noisier.
IF THAT DIDN'T WORK: If the sound is a loud sloshing (like a water bottle), the internal drain pan might have excess water. Try draining the tank as per the shutoff fix above.
HEADS UP: Distinguish this from a grinding or rattling noise. Gurgling/trickling = normal refrigerant. Grinding = a foreign object in the fan or a failing motor bearing. Rattling = loose panels or an unstable window bracket.