With the growth of the EV/HV market, the main cause of cabin noise has changed from engine driving sound to air conditioner noise. The blower noise is the largest in the air conditioner noise, and the noise reduction is urgent. Separated and reattached flows between fan blades are considered to be the main sources of blower noise. In the past, we tried to reduce the noise by reducing the separation. This time, the blade shape to further reduce the separation was produced and evaluated. As a result, the noise was greatly reduced, but a new problem was found that there was a flow velocity condition in which the noise increased despite the small separation. Therefore, we visualized the flow between blades by PIV, investigated the state of separated and reattached flow in detail, and investigated the factors related to noise increase and decrease by measuring noise and pressure fluctuation of blade surface simultaneously. As a result, it was found that the noise generation condition in the separation reattachment flow between blades is not only the size of separation but also the distance of separation shear layer from blade surface and the strength of vortex generated in shear layer.