To forecast and to minimize the noise from wind turbines the title brochure has been issued by Holland Windturbine BV within the framework of the Dutch National Development Program for Wind Energy. In this brochure the contributions of aerodynamic and mechanical noise sources are quantified by means of relatively simple equations. The purpose of the title project is to validate the formulas for mechanical noise. The sources of mechanical noise as given in the brochure are: air noise via ventilation openings, air noise and contact noise via the nacelle wall, and contact noise via the mast. For five wind turbines these sources have been quantified by means of calculations and by means of measurements. It is concluded that the brochure overestimates the noise contribution of the openings, the contribution through the nacelle has been estimated fairly well, but the noise contribution of the mast is estimated badly. The brochure does not give a reliable insight into the degree of importance of the different mechanical noise sources with respect to each other. 22 figs., 4 ills., 7 tabs., 3 refs.