Strange intermittent noise coming from automatic Mini Cooper S
 

We've noticed a strange intermittent humming/vibrating noise that sounds a little like mobile phone interference with speakers or a fax machine. It doesn't seem to happen when the car is cold - takes about 15 minutes of driving to usually show up and I think it might only be there when the car is slow or stopped.

It lasts maybe a minute then goes off again. It's not very loud either but you can feel a corresponding subtle vibration in the car - at least I can through the steering wheel.

Here's a video where the sound is present for the first 25 seconds before it disappears. This was after a 20 minute drive. Outside temperature was about 7 degrees (London).
Any ideas what it might be? It's not the air con, I've ruled that out.