Abstract: Flapping flight strategy is widely adopted by insects and birds, which has drawn considerable attentions due to its high performance. It is worth noting that the good performance and great agility are also achieved with low noise. To apply this flight strategy to engineering, it is necessary to conduct corresponding study to understand both the aerodynamics and acoustics of the flapping wing. In this paper, an immersed boundary method was used to numerically study the sound generation by flapping wings in hovering flight. Both rigid and flexible wings were considered. The flexibility and structure-to-fluid mass ratio were varied to study their effects on the aerodynamics and acoustics. The results show that the rotating motion of the flapping wing can increase the lift and efficiency, and reduce the sound generation. The near field sound generated by the flapping wing in hovering flight is significantly influenced by the vorticity shed from the wing, especially at larger rotating amplitude. A medium flexibility improves the aerodynamic performance, i.e., increasing the lift generation and efficiency. In addition, it is found that a lower sound can be achieved at a structure-to-fluid mass ratio of 1.0 and dimensionless flapping frequency of 0.3~0.4 with the high efficiency conserved.