Peugeot 308 Hissing Noise

I had something very similar happen on my 308. That hissing is a telltale sign. Check the intake hose that runs from the air filter to the intake manifold. I bet you'll find a crack or split in it somewhere, especially with that mileage. The check engine light and fuel consumption issues make sense with a leak there. Is the hissing constant, or only under certain conditions?

Thanks for the tip! I'll give that hose a closer look tomorrow. When you say "intake hose", are you referring to the one directly after the airbox, or further down towards the turbocharger? The hissing seems to get louder as the engine revs increase, especially noticeable when accelerating uphill or under load.

Yep, the one right after the airbox is where mine was leaking. Because the engine works harder when accelerating uphill, it creates a bigger vacuum, which makes the leak more apparent. It's worth getting it checked at a workshop to make sure there aren't any other issues with the intake manifold or turbocharger itself but I am 99% certain that hose is the culprit. Good luck!

I checked the intake hose thoroughly today and found a split on the underside, just as you said. Replaced it, and the hissing is gone, the check engine light is off, and my fuel consumption seems to be back to normal. Cost me 100€ for the hose and a bit of elbow grease. Thanks a lot for your help!