Peugeot Partner Air Intake Leak

The symptoms you describe indeed point towards an air intake leak. I encountered a comparable situation with my old Volvo V70; different make and model, but the underlying problem was quite similar. Have you inspected the intake hoses, particularly the one leading to the turbocharger, for any visible cracks or loosening of connections? A faulty vacuum hose can also lead to such issues. Is the hissing noise constant or does it fluctuate with engine load?

I have given those hoses a look, but I'll admit, it was a quick once-over. I'll get a better look at them this evening. The hissing does seem to get louder as I accelerate, especially when the turbo kicks in. When you say vacuum hose, which one are you referring to specifically? Any tips on pinpointing the exact location of the leak?

Concerning the vacuum hose, it's the one connected to the intake manifold responsible for regulating various engine functions. A thorough inspection, ideally with smoke testing equipment, is required to pinpoint the precise leak location, visual inspection is not enough. Given your observations and the potential for further complications, such as damage to the turbocharger due to improper air/fuel mixture, I would recommend taking it to a trusted workshop for a professional diagnosis.

Appreciate the advice. A closer inspection confirmed a split in the main intake hose, right where it connects to the turbo. Replaced it, cost 100€ all in, and all the symptoms vanished. Thanks again for pointing me in the right direction!