Random walk in a percolation cluster was studied in the simultaneous application of both DC bias and AC driving fields by Monte Carlo calculations. Amplitude and phase shift of the random walk response to the external field of B0 + B1 sin omega-t were calculated as a function of B1, omega and B0. Some new observations are made, which cannot be expected simply from the linear superposition of the two separate results for a constant bias field (B0) and an AC driving field (B1 sin omega-t) respectively.