It was a question that stumped me for a few minutes, then I remembered the digital-sounding interference recorded when using my Sony Cybershot camera while recharging.
To minimise other noises I covered the microphone on the camera with Blu Tack, on tape to ensure it wouldn't get stuck in the microphone holes. Two takes of around six minutes were recorded, which is the maximum possible on each side of my old memory stick.
These were cut into three and layered in Ableton Live. I panned them in various directions and removed the one with the most noise from elsewhere in the house. Then equalised them before running through an effect to widen the sounds.
The image below comes from a Reddit thread about a pregnant woman emanating WI-FI.