Koleos Intake Manifold Flap Control

I had similar symptoms in my Koleos. Have you checked the air intake system for any obvious leaks or obstructions? Maybe a split hose? Is the hissing constant, or only under specific conditions?

Good shout! I've visually inspected the intake, nothing obvious. The hissing is definitely tied to acceleration; the harder I push, the louder it gets. Could a dodgy intake manifold flap control switch really cause all this chaos?

Yes, a faulty intake manifold flap control switch can absolutely cause those issues. The engine will be getting incorrect air mixture and is struggling. Based on your description, I would advise a professional inspection; sounds like a workshop visit is in order.

Cheers for the confirmation! I was dreading that. I took it to a shop; they diagnosed the intake manifold flap control switch. Quoted me 465€ for the repair. Ouch! But at least I know what's up. Thanks again for the help!