The dynamic instability of vibrating systems withviscous damping and having one degree of freedom subjectedto parametric excitation under square wave input isstudied using numerical method.The system considered is thus of the general form:..Aq + cq + [K + Qt(t) Jq = 0where Qt(t) is of stepwise form.under different test conditions. The results indicate that the average normal forcehas a significant effect on generating noise under self-excited vibration; differentnoise levels are observed as the average normal force is changed. For the sameaverage normal force range the~xperimental data demonstrated that, introducingexternal excitation to the system decreased both the average coefficient of frictionand sound pressure levels in comparison with the results of self-excited vibrationcase.