At part load (at lower flow rate than the design flow rate), the flow pattern of backward curved, housingless centrifugal fans gets more and more unstable (unsteady) with sinking flow rate. In direct consequence, both a significant increase in noise emission, appearing especially as distinct subharmonic humps in the sound power spectra, and a decrease of the fan efficiency can be observed. By means of unsteady CFD simulations, which are validated by experimental results, the mechanisms for this undesired behavior have been investigated. It turns out that the phenomenon is self-induced by the impeller and does neither correlate to any upstream turbulence or flow inhomogenouities, nor any influence of the flow in the leakage gap can be identified. Instead, it seems to be induced by flow structures located on the downstream side of the impeller, where strong vortical structures, rotating in circumferential direction at a rotation speed with slip to the impeller, can be observed. These vortical structures are fed by detaching vorticity of highly loaded impeller blades, and they grow and change their structure with sinking flow rate. It was found out, that by controlling and partly avoiding these pressure-side rotating flow inhomogenouities, the part load behavior of the fan can be significantly improved at low flow rates. The result is that the impeller efficiency and the pressure rise can be enhanced at the corresponding duty points, whereas the noise generation can be strongly reduced. Thus, the operating range of the fan can be significantly extended towards lower flow rates. The numerical results are confirmed by some experimental results.