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Abstract: A installation caused severe hum, with 30 Hz as dominant frequency, causing serious annoyance. The acoustic study showed resonances around 30 Hz. Poor flow conditions existed at the inlet and outlet of the forced draft fan. Flow patterns were studied using CFD. Both the inlet and outlet contained large vortices, causing the strong hum. Changes in the design were studied. The finally changes consist of changing the shapes of the inlet and outlet duct, in combination with several flow guiding vanes. Additional silencing was designed with the acoustic module. After completion of the changes, preliminary noise measurements showed 7 dB reduction. These improvements significantly reduced annoyance in the neighbourhood.