Trafic diesel rough running hissing sound

I had a similar problem with my Trafic, although I have petrol engine. The symptoms sound very familiar – reduced power, rough running, and a hissing noise. It could well be the intake manifold flap. Do you notice any difference in performance when the engine is cold versus when it's warmed up?

Thanks for the quick response. To answer your question, the engine seems to run slightly better when cold, but the problems are still present, just maybe a bit less pronounced. Could you elaborate on what exactly was wrong with your intake manifold flap?

Okay, that sounds very much like what I experienced. The flap wasn't opening and closing properly, restricting the airflow. It’s difficult to diagnose without proper equipment. Given your symptoms, it's probably best to get it checked out by a workshop. They can properly diagnose the issue and confirm if it's the manifold flap or something else.

Thanks a lot for the advice. I took it to a local mechanic, and after diagnosis, the switch for the intake manifold control was indeed defective, they quoted 465€ for the full repair. I got the flap control replaced, and everything is running smoothly again.