If the PSA 2.0d has a vacuum controlled VNT turbo like the 1.6, the vacuum control of the VNT mechanism is the most likely cause but it could also be a minor boost leak.
If you locate the turbocharger in the engine bay there will be a small hose connected to the VNT actuator. If you have someone start the engine whilst you watch the actuator, the actuator arm will move from one end of its travel to the other; If the arm struggled to move the all the way or it doesn’t stay still in position and moves up and down there is a problem. If you follow the hose back it will lead to the vacuum control valve which is normally a small brown or black device with a pierburg logo, an electrical plug and some other hoses leading to the vacuum pump on the engine. You can diagnose this system by checking the hoses are intact and none have come off. To check the system holds vacuum you need a vacuum pump (like a mityvac, you can get cheap lookalikes on ebay/amazon), it can’t be properly tested by pressurising the hoses as sometimes hoses can collapse or pinch while under vacuum which will also cause problems. The vacuum control valve has a small foam filter inside that can be cleaned (videos on youtube). If the hoses are fine and cleaning the foam filter doesn’t help the valve itself may be faulty.
As for a boost leak you will hear some whistling under load which might be very faint. It might help to drive around with the windows open and have a listen. Sometimes intercooler hoses can be burst or slightly loose but will only properly leak under full boost pressure. This wont always log a fault code as the difference between desired and actual pressure does not exist for long enough. If I remember right you can search fault codes on VIDA such as an under-boost code and it will tell you what conditions need to be met for the ECU to log it, sometimes the required time can be quite long which won’t pick up slight differences or intermittent problems. In the engine bay you will see a misting of oil around hoses that have leaked as oil vapour from the breather hose travels through the intake system.
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