White noise obtained as the chaotic oscillations in a phase-locked loop (PLL) in which the dynamics is compatible with that of a Josephson junction is studied experimentally. Focusing on an analogy between the PLL and the washboard model, several types of signals to drive the PLL are examined, and the driving frequency dependence of the appearance of unbounded chaotic oscillations closely related to the white noise is investigated. Consequently, it is considered that the washboard structure in the dynamics of the PLL dominates the white noise generation, and the mathematical function of the driving signal affects its parameter dependence. The frequency characteristics of power spectrum dependence of the power spectrum of the chaotic oscillation in the high frequency region is also discussed. Moreover, a frequency modulated driving signal seems to remarkably broaden the parameter region where the white noise is obtained if compare with periodic driving signals.