Lacetti Hissing Sound and Fuel Consumption

Sounds like you've got a vacuum leak somewhere in the intake system. The hissing under acceleration is a classic symptom. The higher fuel consumption and check engine light corroborate that, as the engine is trying to compensate for the unmetered air entering the system. Start by checking all the vacuum hoses connected to the intake manifold and throttle body. Give them a good look and feel for cracks, especially where they connect. Has the car had any recent work done?

Thanks for the quick reply! Vacuum hoses, got it. I'll give them a good look-over. I replaced the spark plugs about a month ago, but that's it. Could I have accidentally knocked something loose while doing that?

It's possible you disturbed something when you were replacing the spark plugs. Double-check any hoses or connections you might have brushed against. Given the symptoms, and the age of the car, I'd also pay close attention to the intake hose itself, the one that connects the air filter box to the throttle body. Those can crack and leak over time, especially at the clamps. Honestly, at that mileage, it's probably worth taking it to a trusted mechanic and having them do a smoke test. They can pinpoint the leak quickly and accurately. I had a similar thing happen with my old car a while back. Turned out it was a tiny crack in a hose, but it caused all sorts of headaches. Saved me a lot of time and guessing.

Thanks a bunch for the advice! I checked the hoses as you suggested and found a crack in the intake hose. Taped it up for now and the hissing is gone! I'll order a new one. The mechanic replaced it for 100€. Hopefully, my fuel economy will be back to normal. I appreciate the help!