Peugeot 207 Hissing Noise and Fuel Consumption

I had something similar happen with my 207. Check all the hoses connected to the intake manifold, especially the one running from the air filter box to the engine. Pay close attention to where the hose clamps are. Is the hissing sound constant, or does it change with engine speed? Any rough idling or misfires?

Thanks for the tip! I'll definitely check those hoses. The hissing gets louder as the engine revs higher. As for the engine, it seems to be running okay at idle, though I think I felt a slight misfire yesterday. What exactly was leaking on yours?

Sounds very much like the intake hose. Mine had a split on the underside, near the manifold. It's a common issue, apparently. You can try to find it yourself, but honestly, with those symptoms, it's best to have a workshop do a proper leak detection. They'll be able to pinpoint it quickly and prevent any further engine damage.

Okay, thanks a lot for the advice! I took it to the workshop, and you were spot on – it was a split intake hose. They replaced it, and the hissing is gone, the check engine light is off, and the fuel consumption is back to normal. Cost me 100€. Thanks again for pointing me in the right direction!