In-band crosstalk, due to multiple interferers, is one of the most severe physical impairments in optical transparent networks. Differential phase-shift keying (DPSK) has been identified as an attractive modulation scheme to be used in such environments due to its robustness to in band crosstalk. At the output of the receiver photodiode, which is assumed to behave as a square law device, the in-band crosstalk interferes with the signal, resulting in the signal-crosstalk beat noise and the crosstalk-crosstalk beat noise. Usually this last noise contribution is neglected, but in this paper the impact of the crosstalk-crosstalk beating terms is considered and quantified. It is concluded that these terms have a growing influence as the optical signal-to-noise ratio (OSNR) value increases.