Crown Victoria Hissing Sound

Sounds familiar. I had a similar issue with my Crown Vic. That hissing sound is a big clue. Have you checked the intake hose that connects the air filter box to the intake manifold? Those hoses are notorious for cracking, especially with age and heat cycles. Give it a good look over, feel for any soft spots or splits. Also, check the connections to make sure they are tight. It could also be the intake manifold gasket itself, but start with the hose. That's the easiest and cheapest thing to check.

Thanks for the quick reply! I did take a quick look at the intake hose, but didn't see anything obvious. Where exactly should I be looking for cracks, and is there anything else I should check while I'm at it? Is it possible to fix it myself?

Okay, so when I had the problem, the crack was on the underside of the hose, where you couldn't easily see it from above. You pretty much have to get your hand under there and feel around. Also, check where the hose clamps onto the throttle body and the air filter box; those are common failure points. Honestly, even if you find a small crack, it's probably best to just replace the whole hose. As for fixing it yourself, you could try patching it with some tape, but that's just a temporary fix. The best solution is to replace it. It's not a complicated job, but if you're not comfortable doing it, a mechanic should be able to do it pretty quickly and cheaply. I think I ended up paying around 100€ at the shop.

Thanks a lot for the advice! I checked the intake hose again like you suggested and found a crack on the bottom side, just like you said. I took it to the workshop and they replaced the hose. Indeed, the final bill was 100€. The hissing is gone, and the check engine light is off. Fuel economy is already improving. Appreciate the help!