Fiat Idea power loss hissing noise

Sounds like an intake issue. The symptoms you describe could definitely point to problems with the intake system. Have you checked for any obvious leaks in the intake hoses or around the manifold? A split hose could cause hissing and loss of power. Is the 'check engine' light on?

I haven't spotted any obvious leaks, but it's hard to tell for sure. The 'check engine' light is on, yeah. Could it be something like the intake manifold flap control? I've read that can cause similar problems.

Yes, a faulty intake manifold flap control switch could be the culprit, especially given the symptoms you're experiencing and the 'check engine' light. The flap controls the airflow into the engine, and if it's not working properly, it can lead to reduced power, rough running, and increased fuel consumption. Considering the age and mileage of your Idea, it's a possibility. I'd suggest getting it checked out at a workshop. They'll be able to properly diagnose the problem and confirm if it's the flap control or something else entirely.

Thanks for the advice. Took it to the workshop, and you were spot on – it was the intake manifold flap control. Cost me 465€ to get it sorted, but it's running much better now.