Diagnosis
That's the refrigerant moving through the system. It's normal, especially during startup or when the unit tilts.
How to Fix It
Ensure the unit is sitting completely level on the floor. If one of the casters is on a rug edge or debris, it can tilt the internal compressor and make the refrigerant flow noisier. Adjust its position.
If That Didn't Work
The noise could be water sloshing if the internal drain pan has condensation buildup. In dehumidifier mode, this is more common. Make sure the continuous drain hose is not plugged if you're using it.
Heads Up
A loud grinding or screeching metal sound is NOT normal gurgling. That indicates a failing fan motor or compressor and means you should turn the unit off and seek service.