Diagnosis
That's the sound of the compressor slugging liquid refrigerant, usually because the unit was moved or tilted while running or shortly after.
How to Fix It
Turn the unit OFF with the power button, wait 5 full minutes for the refrigerant to settle, then turn it back on. The noise should stop. If you just moved it, this is almost always the fix.
If That Didn't Work
The noise is coming from the condensate pump. Unplug the unit, remove the bucket, and check for a small plastic piece or debris lodged in the pump impeller. Gently clear anything you see.
Heads Up
Always let the unit sit upright and powered off for at least 10 minutes after any transportation. Tilting it can flood the compressor with oil and refrigerant, causing this noise and potentially long-term damage.