One possibility is that the 'white noise' is generated within the radio receiver and only becomes apparent when the agc turns the gain up. My guess is that there are two consecutive rf gain blocks each with their own agc loop. The 80Hz modulation of the carrier forces the first agc loop to turn the gain down, then the lack of signal to the second block forces the second agc loop to turn the gain up and amplify white noise. Can't prove this without seeing the schematic for the receivers.
PS - Don't knock DERG too much, it's kinda lonely up there in Durham, especially if he's out on the moors.