Laguna diesel power loss hissing noise

The issues you have described are certainly indicative of a problem within the air intake system, specifically concerning the intake manifold and its associated control mechanisms. The hissing noise you hear could very well be a vacuum leak around the intake manifold or a related component. The jerking and reduced power suggest that the engine isn't receiving the correct amount of air, or that airflow is being disrupted. The increased fuel consumption would corroborate this. Have you inspected the manifold for any visible cracks or damage, and have you checked the vacuum lines connected to the intake manifold flap control for leaks or disconnections?

Thanks for the detailed response. I haven't spotted any visible cracks or obvious damage to the manifold itself, and the vacuum lines seem okay at first glance, but I haven't done a thorough check with a vacuum gauge or anything like that. Is there a reliable way to test the intake manifold flap control switch itself, maybe with a multimeter, to confirm if it's faulty? I am not sure where to start.

Considering your symptoms, your mileage, and confirmation of no obvious external damage combined with my expertise, the intake manifold flap control switch being defective is the likely culprit. While it may be testable with a multimeter, such testing involves accessing vehicle-specific diagrams and understanding acceptable electrical values, and potentially operating the mechanism while testing. Quite honestly, given the importance of proper airflow management in modern diesel engines, and lacking the appropriate diagnostic equipment and repair information, I would advise taking the vehicle to a qualified workshop for a proper diagnosis and repair. It is highly likely a faulty switch will require replacement, which may also need adaptation by diagnostic equipment. A repair of this nature may cost in the region of 465€.

Thanks again for your expertise. I suspected it was beyond my capabilities without specialized tools. I appreciate the advice; I'll book it into a workshop for a professional diagnosis and repair. Your cost guestimate turned out to be spot on.