The sound field generated by the interaction of spatial instabilities on the shear layer shed from a duct with the nozzle lip is studied. It is shown that the intensity varies with direction θ from the exhaust and with the subsonic exhaust speed U according to I ∼ U6(1 − cosθ)2 and I ∼ U6 sin2 θ for the axisymmetric and first azimuthal (sinuous) modes respectively. The first of these results is interpreted in terms of monopole and dipole sources at the exit plane, representing the acoustic effect of fluctuating mass flow and axial thrust across the exit plane, and the second in terms of a transverse dipole at the exit plane, corresponding to fluctuations in cross-stream thrust. A correlated thrust fluctuation of 1% is shown to overwhelm the jet mixing noise in the forward arc, θ > 90° while the acoustic efficiency of the interaction process is never less than 10−6M3 even under the cleanest possible exit conditions. Forward flight of the duct a t Mach number Mα is shown to increase the forward-arc intensity by the factor (1 + Mα cos θ)−4. It is suggested that much of the discrepancy between the noise fields of real engines and the predictions of Lighthill's theory of jet mixing noise – the so-called ‘excess noise’ problem – can be explained in terms of this interaction mechanism.