DIAGNOSIS This is normal operation. The sound is refrigerant circulating through the coils.
FIX No fix needed. The gurgling or bubbling is most noticeable when the compressor first starts or cycles off. It's a sign the refrigeration cycle is working.
IF THAT DIDN'T WORK If the sound is a loud knocking or rattling, that's not normal. Check that the unit is on a level floor and that the water bucket is securely seated.
HEADS UP The sound may be more pronounced if the unit is working in a very humid environment, as it's moving more refrigerant.