Mercedes ML diesel power loss hissing sound solved

The symptoms do point towards a potential issue with the intake manifold flap control. However, it could also be a vacuum leak affecting the intake system. Have you checked for any obvious cracks or disconnections in the vacuum lines? Also, is the hissing sound constant or only during acceleration?

I've given the vacuum lines a quick once-over, but nothing jumped out. I'll take a closer look tomorrow. The hissing is definitely only when I'm accelerating, especially under load. Could a faulty manifold flap really cause that much of a racket and power loss?

The hissing sound during acceleration, coupled with the other symptoms, does strengthen the case for a faulty intake manifold flap. The flap not operating correctly could severely restrict airflow. Though, I still suggest caution. To be completely sure, it might be best to get a professional diagnosis. A workshop will likely be able to confirm if it is indeed the manifold flap and if there are any associated issues.

Appreciate the advice. I bit the bullet and took it to a workshop. Turns out you were spot on – it was indeed the intake manifold flap control! They replaced it, and everything is running smoothly again. Cost me 465€, exactly as I feared, but at least the old girl is back to her former glory. Thanks again for your help!