ML Hissing Sound and Fuel Issue

Hissing sound and poor fuel economy on an ML? Sounds familiar. I had a similar problem with my ML, though I can't remember the exact mileage. Check the intake hose that connects to the turbocharger. A split or crack there can cause exactly what you're describing. The check engine light is probably on because the engine is getting unmetered air after the mass airflow sensor. Have you checked for any obvious cracks or loose connections around the intake?

Thanks for the tip! I'm not super mechanically inclined, so could you tell me where exactly I should be looking for this intake hose? Is it easy to spot, or is it buried deep in the engine bay?

Okay, so it's usually pretty visible once you pop the hood. It's a large hose, typically black, that runs from the air filter box to the engine, near the turbocharger. Just follow the path from the air filter, and you should find it. But honestly, if you're not comfortable poking around, I'd recommend taking it to a workshop. A torn intake hose is a common issue and they should be able to diagnose and fix it pretty quickly.

You were spot on! Took it to my local guy, and it was indeed a split in the intake hose. They replaced it, cleared the check engine light, and the hissing is gone. Fuel economy seems to be back to normal too. Total cost was around 100€. Thanks for pointing me in the right direction!