In this study, tonal noise produced by an isolated centrifugal fan is investigated using unsteady Reynolds-averaged Navier-Stokes (URANS) equations. This type of fans is used in ventilation systems. As the fan propagates tonal noise in the system, it can severely affect the life quality of people that reside in the buildings. Our simulation shows that turbulence kinetic energy (TKE) is unevenly distributed around the rotation axis. Large TKE exists near the shroud at the pressure sides of the blades. It is caused by the recirculating flow. Moreover, the position of the largest TKE periodically varies among the blades. The period corresponds to approximately 4 times the fan rotation period, it was also found in acoustic measurements. The magnitude of the tonal noise at the blade passing frequencies agrees well with experimental data. By analyzing the wall-pressure fluctuations, it is found that the recirculating flow regions with large TKE are dominant sources of the tonal noise.