Noise generated by a gas turbine engine 6 supported by a pylon 8 on a wing 2 of an aircraft is reduced by influencing a shear layer generated between the free-stream flowfield and flow from the engine 6. The shear layer is influenced by means of one or more winglets 18, 20, 22 which interact with the free-stream flowfield to deflect the shear layer 14 downwardly, to avoid interaction with a flap 4 on the wing 2, or to reduce the strength of the shear layer 14.