Thanks for all the replys.

The noise, when it happens, is white noise like an untuned TV or radio and is a constant volume (loud!!). When and how it manifests seems to be random;

The noise can be steady state white noise, or switching cyclically between white noise and silence with seeming regular intervals, or a very short blast of noise followed by silence.

I have not tried to determine if it is comming out of all 4 speakers, I'll try that if I get an opportunity.

It can happen at first switch on of the car, and thus empeg, before the boot process has completed, and before any music has started to play. It does not appear to be related to any particular track, playlist, album, or artist, or to the movement of the car. Some days, like today, everything was perfect, another day it can happen 3 or 4 times in a short trip.

As I mentioned before I cannot recall it ever happening in radio mode.

I have tried switching off auto volume, loudness, and flat lining the equaliser (very dull) but to no avail.