Diagnosis
Normal operation. That’s refrigerant moving through the coils and capillaries inside the sealed system.
How to Fix It
No fix needed. The sound is more noticeable on larger pint models like the DH70W because they move more refrigerant. It will often change pitch as the cycle starts and stops.
If That Didn't Work
If the sound becomes a loud hiss or screech, that indicates a refrigerant leak. Unplug the unit immediately. Do not use it. A leak means the sealed system has failed.
Heads Up
Do not tilt or move the unit while it’s running. You can force liquid refrigerant into the compressor lines and cause real damage.