Mini Roadster Hissing and Fuel Consumption Issues

That sounds familiar! I had a similar issue with my Mini Roadster. Loud hissing, check engine light, the works. Has the car been feeling sluggish too? Have you checked the vacuum hose connecting to the turbocharger? A leak there can cause all sorts of problems. It might be worth doing some leak detection around the throttle body as well. Is the check engine light still on?

Yeah, the vacuum hose can be tricky to spot if you don't know what you're looking for. It's a small hose, often black, running from the intake manifold to various components. Cracks or splits are the usual culprits. The throttle body is usually mounted on the intake manifold. Honestly, if you're not comfortable poking around, it might be best to take it to a workshop. They can do a proper smoke test to find any leaks. It does sound like the intake hose, and if so, you might be lucky and it will be a cheap fix.

Sluggish yes, but how exactly do I check the vacuum hose? Is it easy to spot, or do I need special tools? And where exactly is the throttle body located?

Just wanted to give you an update. I took the Mini to a workshop, and they confirmed it was a cracked intake hose. Replaced it for around 100€ as you predicted, and the hissing is gone, the check engine light is off, and the fuel consumption seems normal again. Thanks for the help!