Skoda Praktik Diesel Rough Running and Hissing Sound

Sounds very similar to a problem I had with my Praktik, same engine. Reduced engine performance and that horrible hissing sound points to an issue with the intake manifold system. Have you checked the manifold flap? It's like a valve that controls the airflow, and if it's faulty, it can cause all sorts of problems.

The manifold flap, eh? Is that something I could check myself, or am I better off taking it to a mechanic? Is there a way to confirm that's definitely the problem before I start throwing money at it? What did you do to fix it?

Yep, that sounds exactly like what happened to mine. The hissing is a classic symptom. I wouldn't recommend tackling it yourself unless you're confident with car mechanics. You could try to locate the intake manifold and visually inspect the flap, but it's often difficult to diagnose without proper tools and knowledge. I ended up taking mine to a workshop, and they confirmed it was a defective intake manifold flap control. It's a pretty common problem with these engines, apparently. Bit of a pain, but at least it's fixable. I'd advise you to go to the workshop.

Cheers for the advice! Turns out you were spot on. Took it to a workshop, and they confirmed it was the intake manifold flap control. Cost me 465€ to get it sorted, but the Praktik is running like a dream again. No more hissing or jerking!