I have taken the aircraft to altitude and lean cut the engine, then whilst gliding I have wind-milled the prop with the mags off (and they both ground perfectly) sure enough at 2200rpm the white noise kicks in on the radio.
Going back to very early on in the thread, I did mention that a zener diode could, if reverse biased, generate the "white" noise. It seems that condition could exist in the alternator regulator when current to the base of the pass transistor is cut off. A capacitor associated with the zener will need to be open circuit for this to happen, certainly a possibility, and something that a standard regulator test would not identify.