For a start, you might find it’s neither of those things causing the noise. You may have a worn tyre, poor wheel alignment or a faulty brake, to name just a few possibilities. That said, both your suspects are likely contributors to this type of noise.

In any case, there’s a simple way to tell whether a noise is coming from the driveline (engine and transmission) or from one of the wheel bearings. Drive the car at whatever speed is necessary to make the noise appear. If it gets louder as you apply throttle, there’s a good chance it’s driveline related.

But now, with the car still making the offending noise, drop the transmission into neutral (where it’s safe to do so, of course) and let the vehicle coast. This takes all the stress off the driveline and the engine should return to idle. If the noise is still present, then you have a wheel bearing or other problem linked to the wheels, brakes or axles, but not the engine or gearbox.

Holden’s own service schedule for this model Captiva doesn’t specify a change interval for the differential oil. Instead, it recommends the oil be changed when required according to how the car is behaving. Which would suggest that your car is, indeed, ready for new differential oil.

As far as the scan goes, be sure not to confuse a sticking or non-engaging rear differential with the same behaviour from the centre differential. These are both technically differentials, but have vastly different roles to play. In the case of an electronically operated differential, the clutches can sometimes be replaced separately, but in any case, you need to go deep inside the differential, so while your mechanic is in there, have them check out the whole unit.

The V6 engine in the Captiva is well known for its propensity to suffer from worn or stretched timing chains far too early in life. Early signs can be a rattling noise when the engine is first started (hot or cold) as well as a check-engine light on the dashboard. The check-engine light is a result of the stretched chain allowing the camshafts to move so far out of sync with the rest of the engine that the sensor that measures their position can longer 'find' them. At which point, the warning light is triggered.

You can also have the car electronically scanned to see what’s going on. If the computer throws up fault codes P0008, P0009, P0016, P0017, P0018 or P0019, then you almost certainly have a stretched timing chain. Replacement of the chain is the only proper fix.