What causes the loud buzzing noise that one hears from some jet engines, particularly at take-off and high throttle settings? It seems to occur especially in recent-model, large, high-bypass turbofans, like, say, those of a 767.
Yup it's the fan without Inlet guide vanes to support a bearing in front of the rotor. The vanes create a siren effect, while the open face blades with vastly different twist angles create a buzz.