Peugeot 307 Hissing Sound Resolved

Sounds familiar! I had a similar issue with my 307 a while back. Check the intake hose, especially where it connects to the throttle body or intake manifold. They can crack and cause problems. Is the hissing sound constant, or does it change with engine speed?

Thanks for the tip! I'll definitely give the intake hose a closer look, especially around those connection points. As for the hissing, it definitely gets louder and more intense as the engine revs higher. Does that point to a specific area of the hose, in your experience?

Yep, that's exactly what mine was doing. Revving definitely made it worse. Honestly, it could be a small crack anywhere along the hose, but those connections are weak points. If you can't find it yourself, I'd take it to a workshop. It'll save you a lot of time and frustration.

Appreciate the advice! I inspected the intake hose meticulously, and found a split on the underside, right where it connects to the intake manifold – a real pain to see! Replaced the hose, and all the symptoms are gone. Fuel economy is back to normal, no more check engine light, and the hissing is history. Total cost was around 100€ for the part. Thanks again for pointing me in the right direction!