Strange electronic / interference droning noise from 30mph

I've just but this brand new car.

Over the last few days, I've noticed and electronic / interference sound when you reach 30/40 mph in the car.

It's not the radio or audio system because that's turned off, and there's nothing in the car to cause the interference sound.

I'm a little concerned about this as it's cost me a lot of money and I want it to be perfect.

Can anybody give any suggestions to what it may be. I've booked it in at the dealers for them to look at, but what could it be, so they have a better idea of what to look for.