Thanks bSudo for your reply. I picked up my X3 from my local BMW garage today after finally diagnosing the fault. It was the transfer box oil needed changing. I was really surprised and unhappy with this as the car was under 4 years old and had only done 20,000 miles. I was charged £336 that was reduced from £409 because I expressed how unhappy I was with the price and fault. I spoke to one of the technicians he told me that it was a serviceable part and could go at any time but admitted that it was unusual to fail at this low mileage. I am really disappointed with BMW.