All refrigeration works under low & high pressures created by a compressor, it is not unusual to hear strange noises such as:
Bubbling noise: This noise is caused by the flow of the refrigerant in the system especially during the off cycle (compressor not operating) & refrigerant state/pressure.
Water flow noise: Refrigerant flowing into the evaporator caused by change of pressure/refrigerant state.
Air blowing / Fan noise: Normal fan noise, this noise can be heard during normal operation of the system due to the circulation of air.