Hi, hope someone can help, I have a Countryman Cooper d all4. When I get to about 35 mph and above there is a noise a wheel bearing or something growling. I have jacked up and checked each wheel for any play and spun them to try and hear any noise but all appears ok. Someone suggested that perhaps it might be the diff. Should the oil be changed in this as I have done just over 50k. The noise has been apparent for the past couple of months and I really worried that it might not be economic to repair. TIA