The differentials and many other automotive components suffer from wear and tear due to the high mileage of the cars that are driven over the years or the bad use of the same customers who do not respect some basic points.
This wear affects the differential which in the long run causes stress damage to the bearings or in the worst case the bevel gear and planetary satellites that create those famous whistling or rolling noises in the passenger compartment.
So we can diagnose a defective differential.