We report on experimental study of the exciton-polariton emission (PE) polarization noise below and above the polariton lasing threshold under continuous-wave nonresonant excitation. The experiments were performed with a high-Q graded 5λ/2 GaAs/AlGaAs microcavity with four sets of three quantum wells in the strong-coupling regime. The PE polarization noise substantially exceeded in magnitude the shot-noise level and, in the studied frequency range (up to 650 MHz), had a flat spectrum. We have found that the polarization and intensity noise dependences on the pump power are strongly different. This difference is ascribed to the bosonic stimulation effect in spin-dependent scattering of the polaritons to the condensate. A theoretical model describing the observed peculiarity of the PE polarization noise is proposed.