You've almost got it. The 'white noise hiss' is a broadband composite of all audible frequencies. When you change the master volume setting, you change the input impedance of the power stage, which changes the loop impedance of the system. In this particular case, the resonant frequency shifts down. Since the change happens relatively slowly, and since there is so much gain in the system, it is able to 'track' the change in resonance, and keep on howling.