Wings operated at low and moderate Reynolds number such as the ones of UAVs or the blades of small turbine and of compressors, can be the source of an aero-acoustic phenomenon called laminar boundary layer instability noise. The narrow band noise can be attenuated using trailing edge (TE) serrations (Chong et al. 2013, Moreau et al. 2012) but the mechanism behind tonal attenuation is yet to be investigated in detail. An experimental study (1.32X105 < Re < 5.30X105) is conducted in an open-jet low turbulence wind tunnel using NACA 0018 airfoil (c = 0.20m) with modified TE. The acoustic emission in the far field is recorded using far-field microphones whereas the developing field near the TE is studied using Time resolved planar particle image velocimetry and oil flow visualization techniques. Study showed the serration-3 (2h = 20mm, λ = 10mm) has maximum tonal noise attenuation and effect the point of separation of laminar separation bubble. The span-wise change in flow characteristics in case of serrations can be attributed to the effective chord-length of the wing at each span-wise position. Further span-wise correlation of chord-wise flow fluctuations show the flow being turbulent upstream of serration-3 TE modification.