Fan noise prediction and control: On modern high-bypass aircraft engines, the fan blade tips rotate supersonically at take-off and produce a sound rich in harmonics, called buzz-saw noise. Predicting the level and frequency content of this noise in the fan duct is a nonlinear acoustic propagation problem which has been reformulated in the frequency domain with promising results. Of specific interest is the effect of absorbent linings. Other work is underway that is aimed at a deeper understanding of the broadband noise generated by a fan. The possibility is also under investigation of modifying the inlet flow field into a fan with a view to providing "active control" of the tonal noise radiated.