In this paper, a new continual self-tuning control scheme based on an unknown process response under load disturbance is proposed. It can re-estimate the possibly changed process frequency response using the benign process transients under an unknown step or impulse, like load disturbance. According to the disturbance response pattern, the unknown step/impulse like load disturbance is assumed to be generated by passing a step or an impulse through unknown rational function dynamics. With process input and output frequency response computed by FFT, the best estimation of the process frequency response is obtained using the least squares method. Based on the re-estimated process frequency response, a PID controller is re-tuned to adapt to the possible changes in the process. The effectiveness of the method is demonstrated through simulation examples and real-time tests.