DS3 Hissing Noise Air Leak

It definitely sounds like an air intake issue. The hissing noise is a strong indicator. A leak after the mass airflow sensor but before the turbocharger (if you have one) would explain the increased fuel consumption and the check engine light. The hissing is the sound of unmetered air being sucked into the engine. Does the car seem to be misfiring or running rough at all?

Thanks for the quick reply! It does feel a bit rough sometimes, especially at lower speeds. Could it be a vacuum hose or something similar? Where should I start looking?

The symptoms you describe, combined with the hissing, strongly suggest an intake hose leak. Given the age of the vehicle and the mileage (142166 km), the rubber hoses can become brittle and crack. I'd recommend visually inspecting all the intake hoses from the air filter to the engine. A split or crack in one of the hoses is the most likely culprit. It might be tricky to spot without removing the hoses though. Unless you're very familiar with car mechanics, you might be better off taking it to a workshop.

Thanks so much for your help! I took it to a mechanic, and you were spot on. It was a cracked intake hose. They replaced it, and the hissing is gone, the check engine light is off, and it seems to be running much smoother. The repair cost me around 100€. Really appreciate the advice!