What kind of "noise" is normal when the air conditioner works?

A. the wind noise of air conditioning operation can be improved by adjusting the corresponding wind speed.

B. The sound of "crackling" of thermal barrier and cold contraction of plastic parts of air conditioner can not be avoided completely. It can only be improved by adjusting the tightness or position of the shell.

C. The sound of rapid vaporization and liquefaction of refrigerant in the pipeline.