When dealing with urban noise, most of the attention is focused on the typical sources, the ones given by the Environmental Noise Directive 2002/49/EC: major roads, railways, airports and industries. But what happens in protected and quiet areas, like courtyards or parks? They are partly shielded from the previously mentioned sources, but many others might be relevant: maintenance and repair work, glass recycling, loud music, car parking, ventilation and cooling systems, etc. The Sonorus project considers every aspect of the urban sound environment in its holistic approach to the matter. Hence, if non-traffic sources play an important role in quiet areas, they should be analyzed and characterized, in order to evaluate how such sources will affect the environment. This paper presents the outcome of the first stage of one of the research projects within Sonorus, the characterization and synthesis of a fan sound for auralization purposes. The technique used is based on spectral modelling synthesis. The input parameters to the model can be varied in order to simulate various running conditions, such as variable fan speed. The source synthesis is combined with a propagation model in order to complete the auralization process.