C30 Hissing Sound and Fuel Consumption

Sounds familiar! I had almost the exact same symptoms on my old Opel Astra, but with a slightly different engine. Higher fuel consumption and the hissing sound were the first things I noticed. Did you check the intake hose between the air filter and the intake manifold? It might be cracked. Is the hissing constant or does it change with engine load?

Good point about the intake hose. I haven't checked that specifically yet. I'll give it a look. The hissing definitely gets louder as I accelerate, so it changes with engine load. If it's a leak there, is it usually obvious, or could it be a small crack that's hard to spot?

On my Astra, it was a split right on the underside of the hose, where I couldn't easily see it. Because the hissing gets louder with acceleration, that really points to a leak that gets worse under pressure. Honestly, it's probably worth taking it to a workshop; they will have the right tools to properly diagnose the problem.

The advice was spot on! It turned out to be a cracked intake hose, just like you suspected. The shop replaced it, and it cost me around 100€. Fuel consumption is back to normal, no more hissing, and the check engine light is off. Appreciate the help!