Its a deep\low whirring\rumble type of noise that occurs above 25 - 30 mph - From the back of the car
TBH I don't think it is the back box any more as coasted with engine off and noise is still there... So this thread is no longer required
However if you have any idea what the noise could be then I would be grateful for suggestions!
I have checked brakes are not binding and have checked the wheel bearings, which don't *seem* to be worn (no play and no noise when spun by hand)