Mazda Commercial Range

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.

I’d start by talking to a transmission specialist. There’s a fair chance they will have seen these exact symptoms before in this make and model and will know what’s wrong without wasting any time or money on guesswork. It’s probably also worth going back to basics and checking the transmission fluid level, as low fluid can cause all sorts of drive problems. If, on the other hand, the gearbox is worn out internally, you’ll either need to rebuild or replace it, but don’t rule out something less expensive like an internal selector fault or even a torque converter problem.