V Klasse diesel intake manifold issue

That sounds like what happened with my V Klasse. The hissing is a big clue, and the reduced power and rough running are classic. Is the engine light on? Is the hissing constant, or only when accelerating? The flap control directs airflow. If it fails, you get chaos, hence the reduced power and rough running. It could be something else in the intake system.

Yes, the engine light is on. The hissing is loudest when accelerating. Was your issue the flap control switch, or a vacuum leak in the intake?

My engine light was on, and the hissing was load-dependent. The diagnostic pointed to the intake manifold flap control. It could have been a vacuum leak, but the garage said it was the flap motor/switch. With your symptoms and the engine light, a workshop diagnosis is best. It will save time and money.

The workshop confirmed a defective intake manifold flap control. The repair was 465€, including parts and labor. The symptoms are gone.