The automatic transmission is a mysterious, marvellous piece of equipment that manages to work almost in spite of its complexity. Guessing at a problem like this is fraught with danger, so the best advice is to get the vehicle to a transmission specialist, download the fault codes and see if they give the technician any hints as to what’s up.

Transmission problems with this make and model are hardly unknown, so avoid driving the vehicle any farther until it’s been looked at. You may end up doing more damage by continuing to drive it.

The actual problem could be anything from a transmission that is about to catastrophically fail, to one that has simply lost its 'memory' or programming and can be rebooted fairly easily and quickly.

Conventional wisdom says the first place to start looking is the turbocharger and its plumbing. A turbocharger operates at very high speeds and a whistling noise is quite common. What you shouldn’t have, however, is a loud version of that noise, or a whistle that is suddenly louder than ever before.

Cracked or leaking turbo-plumbing can allow the noise to escape, but a turbocharger with a worn bearing can also suddenly become louder in its operation. So that’s where to start looking.

Why does the noise drop in the higher gears? Perhaps because you’re cruising in those gears and not accelerating hard as you do in the lower gears. Less throttle input means a slower turbocharger speed and a drop in turbo volume and pitch.

A trip to your local transmission specialist is the first step here, and don’t even think about driving the vehicle father than that. You could easily do more damage than is already existing. Modern automatic transmissions are incredibly complex things and there’s a lot that can go wrong.

You might be lucky and simply have a low transmission fluid level (although you’ll still need to find and fix the cause of the leak). But, equally, you may have a transmission that is literally on its last legs. The problem is that all these symptoms can show up through all sorts of transmission woes.

Either way, a specialist workshop should be able to diagnose the problem and work from there.