Ok, back from the dealers. They are not sure what the problem is. It could either be the speaker or the amp causing the speaker to make an odd sound. As I have it booked in anyway for them to fit a new ash tray (as mine is stuck in and won't come out) they will investigate this at the same time. I'm in the US for a week from the 11th July so I will leave the car with the dealer for the week. That way if they need to order a new speaker / amp they have time to do that and fix it for when I get back.