The automotive refrigerant system can occasionally exhibit an excessive noise out of air-conditioner (A/C) vents during vehicle’s developments. If the vehicle has been parked for long hours in summer and the A/C system is turned on, sometimes hissing noise is induced by the refrigerant flow. In order to understand the mechanism, a lot of bench and vehicle tests were conducted. However, there is still not enough to understand the physical behavior in detail. Therefore, for the first step, the visualization method to capture the behavior of multi-phased refrigerant flow jet inside the pipe was proposed with a high-speed camera, some light devices and acrylic test piece. In addition, image analysis to quantify the flow regime from a series of observed snapshots. Using proposed method, the correlation study between flow and noise was performed at A/C bench test. As a result, different flow features such as the velocity can be observed in the occurrence of the noise or not. It is also shown that the valve tends to open largely when it occurs. As another approach to clarify the mechanism, co-simulation method was developed with computational fluid dynamics (CFD) and vibro-acoustic model. Pressure fluctuations on the inner wall of the pipe are calculated by CFD and they are imported to vibro-acoustic model as transient pressure load. Structural modes in the evaporator system and acoustic sensitivity in heating, ventilation and air conditioning (HVAC) are calculated and then the frequency characteristics are revealed. Lastly, from the perspective of both fluid and vibro-acoustic field, some countermeasures for its reduction are discussed.