Mini Countryman Hissing Noise and Check Engine Light

Hissing sound when accelerating and a check engine light? Sounds like a vacuum leak alright. Could be a cracked vacuum hose somewhere in the intake system. Have you checked the hoses around the intake manifold and throttle body? That could definitely cause higher fuel consumption. Is the car a hybrid? I didn't even know they made a hybrid Countryman!

Thanks, that's helpful. Where exactly should I be looking for this vacuum hose? Is it something I can easily see, or is it buried deep in the engine?

Okay, so you're probably looking for a small, rubber hose connected to the intake manifold or the throttle body. Sometimes they connect to the turbocharger system too. It's not always easy to spot, but if you listen closely when the engine is running, you might be able to pinpoint the location of the hiss. Honestly, with the check engine light on, it might be worth taking it to a workshop. They can do a smoke test to find the leak quickly.

Just wanted to give an update. The hissing noise was indeed a leaking vacuum hose. Replaced it, and the check engine light is now off. Fuel consumption seems normal again. Total cost was around 100€. Thanks for the help!