I can hear sounds of air sucking when the turbo is under load on my Mitsubishi Triton 2011. Do you have advice on this issue?

The sound of air hissing on a turbocharged engine is often down to a leak in the intake trunking that takes air from the turbocharger, through the intercooler and into the engine. As this air is under pressure (from the turbo) you’re likely to hear it above the rest of the engine noise. If it’s only a small leak, you might not notice the fall-off in performance (or boost as you’ve described it) but it will only get worse as the damaged part of the trunking deteriorates further. Eventually, you’ll start to feel the leak’s effect as it allows more and more air to escape rather than being forced into the engine.

Checking the boost pressure with an accurate gauge will tell you if any boost is going AWOL, as can an electronic scan which might throw up a low-boost fault-code. But if you can hear the noise and have found the leak, replacing that section of plumbing is the logical first step. If that makes the noise go away, you’ve solved the issue.

Sometimes the leak is down to a split in the trunking, other times it can be as simple as a clamp that has been loosened by the constant pressurisation of the system every time the turbo boosts up. But it needs to be fixed as it can throw the air-fuel mixture way off target and lead to internal engine damage. If you can’t find the leak but suspect that there is one, a specialist workshop can perform what’s called an EVAP smoke test where the intake tract is filled with smoke. At the point where the (visible) smoke escapes, there’s your leak.