The broadband noise sources are investigated on an isolated low-speed fan typical of engine cooling systems. Reynolds-averaged Navier-Stokes (RANS) simulations have been performed on a single blade passage for several flow rates at the same rotational speed. The flow structures responsible for the different noise contributions are identified by a systematic analysis of the simulation results. The aeroacoustic noise predictions are based on Amiet's model for rotating sources in free-field. The contribution of the turbulence-interaction noise and the trailing-edge noise are considered by the appropriate isolated blade response and statistical model of the turbulent sources. The flow parameters of the aeroacoustic response and the turbulent models are extracted from the RANS simulations. The radial evolution of the flow parameters for the different flow rates is analyzed and related to the three-dimensional flows in the machine. The acoustic predictions are validated with experimental spectra measured upstream of the fan in a reverberant room. The two considered mechanisms evolve differently with the flow rate. The leading-edge sources are dominant at low flow rate up to the design point and the self-noise becomes dominant at high flow rate for which the secondary flow structures are limited.