Laguna Hissing and Fuel Consumption

Sounds familiar. Had similar symptoms on my old Saab 9-3. Did you check the intake hose for cracks? A leak there would explain the hissing and the engine misfire.

Intake hose... where exactly should I be looking? Is it easy to spot, or do I need to remove the air filter first?

Yeah, start by checking the vacuum hose and the air filter connections. If you're not comfortable doing it yourself, a workshop should be able to run a leak detection test pretty quickly. It's probably a cracked intake hose.

Thanks. Turns out you were spot on - intake hose leaking. Replaced it for 100€. Runs like new!