Mercedes GLE Hybrid Hissing and Fuel Economy Issue Resolved

Whoa, higher fuel consumption, check engine light and hissing? Sounds like you've got a decent vacuum leak somewhere in the intake. Since you hear hissing during acceleration, I'd bet it's post air filter, somewhere between the turbocharger and the throttle body. That would explain the poor fuel economy and the check engine light, likely a lean code. A cracked intake hose is prime suspect. Check all those hoses thoroughly, especially around the connections. A visual inspection is key, but sometimes you gotta feel around for cracks that are hidden. If the leak is small enough the engine might compensate at idle, but when you put your foot down and the turbocharger spools up, the pressure difference amplifies the leak causing the hissing.

Thanks, that's really helpful! When you say 'post air filter', does that mean I should be looking at the hoses closer to the engine rather than the ones right after the air filter box? And is there a way to tell for sure if it's a vacuum hose specifically, or could it be something else entirely?

Yep, 'post air filter' means between the air filter and the engine itself. Start by closely inspecting any hoses connected to the turbocharger. A vacuum hose leak is a strong possibility, but a split in any of the intake hoses after the turbocharger could also cause the same symptoms. Honestly, with those symptoms, and without getting too deep into diagnostics, I'd recommend taking it to a trusted mechanic. They can do a smoke test to pinpoint the exact location of the leak. It's usually a quick fix, but you don't want to be chasing your tail replacing parts unnecessarily.

Thanks a lot for the advice! I took it to a mechanic, and you were spot on - it was a cracked intake hose near the turbocharger. They replaced the hose, and the hissing is gone, the check engine light is off, and my fuel economy is back to normal. The whole repair cost me about 100€ Appreciate the help!