I changed my diff oil to SAF-XA and what do you know, the noise has completly gone. I cant believe that Ford put the wrong oil in, gives me the sh*ts. I hope that driving around with the wrong oil in the diff for 10 months didnt damage anything. I was going to take my car to Ford to get the clutch replaced, definately not now. I recommend this Castrol fully synthetic oil, the handbook even says to change the oil every 7500km in frequent or prolonged high speeds.