It’s certainly strange behaviour and your theory that because it works perfectly some of the time, it’s probably not terminal is a decent one. If something internal was broken or worn out, the transmission wouldn’t work at all. Which leads any mechanic to suspect you have either a gear selection problem or an electrical glitch.

Scan the car electronically to see if any fault codes are thrown up. But also check for things like bad earths or blown fuses and relays. You can’t rule out a major internal failure until you get inside the gearbox itself, but starting with these simple things can help eliminate other possibilities. You may find the ECU (the computer than controls the driveline) is the culprit, and is having random melt-downs.

You haven’t told me which engine is fitted to your car, but if it’s the turbo-diesel engine, then the cause may be a pretty simple one. This engine has a habit of splitting the plastic trunking that takes the pressurised intake air from the turbocharger to the engine’s inlet manifold. If that happens, the air is free to escape to the atmosphere, and doesn’t make it’s way into the engine.

That’s why you’ll hear a noise like air escaping (because that’s precisely what it is) as well as a lack of power, because the engine is not receiving all the boost from the turbo. You’ll probably also notice that the car is blowing more black smoke than it should.

The fix is a new piece of plastic pipe that should be well within the abilities of any workshop to fit.

Technically, the Captiva operates in all-wheel-drive all the time. But there’s a centre differential that apportions the drive between the front and rear axles to allow individual wheels to rotate at different speeds. If that centre coupling isn’t working properly it could cause the noise you’re hearing, possibly from the tyres that are skidding or skipping as they turn, or mechanical noises caused by what’s known as 'axle wind up'.

The Captiva normally operates much as a front-wheel drive vehicle and only engages the rear axle when the computer detects any wheel slip or loss of traction.