Hi to all that responded.
Nothing new is running in the background, that hasn't always been there.
The graphic first appears on the main options screen (Mission, Mission Editor, Campaigns etc.) and then stays there regadless of what I do. The pattern that's produced follows the position of where the sound originates.
I've disabled/quit every app that's shown in Task Manager to no avail.
The history is that it was working fine, but due to lack of storage space, the latest update failed. I then transferred the entire ED folder to a new drive, and the graphic appeared when I ran DCS. I then removed the app, and deleted the entire ED folder. I then reinstalled DCS from scratch, but the graphic remains!