Abstract: The tonal noise generation from a NACA0015 airfoil at a low (Re_c=1×10⁴) and a moderate (Re_c=5×10⁴) Reynolds number is studied by performing two-dimensional direct numerical simulations. The effects of Reynolds number result in distinguishing features on the acoustic fields and thus the spectra. For the moderate Reynolds number cases, the vortex shedding on the suction and pressure side of the airfoil contributes separately to the generation of the acoustic waves, corresponding to an individual hump with different discrete frequencies in the acoustic spectra. A linear stability analysis is used to clarify the relation between the formation of vortices with the growth of unstable Tollmien-Schlichting (T-S) waves. Results suggest that the single tone observed in the low Reynolds number cases is probably associated with the inherent hydrodynamic instability of the boundary layer, whereas the multiple tones in the moderate Reynolds number cases are considered to be associated with a feedback loop mechanism between the formation of vortices and the excitation of acoustic disturbances. Feedback loops actually exist on both sides of the airfoil rather than just on the pressure side or on the suction side of the airfoil.