The dynamics of an incompressible forced mixing layer formed downstream of a splitter plate were investigated. In the regions of zero growth of the mixing layer, spectral analysis of the velocity time fluctuations revealed the existence of locations in which the time variations of the velocity are quasimonochromatic. In these locations the power spectral density distribution presents sideband frequencies associated with the most amplified frequency component, i.e. the forcing frequency and its harmonics. the sideband frequencies are symmetrically placed with respect to the carrier. The phenomenon also persists with forcing the layer at a frequency equal to either the lower or higher sideband frequency.