Seat Ibiza Diesel Engine Trouble

Sounds like a potential boost leak in your turbo system. I had a similar issue with my Seat Leon. Check the vacuum lines and the intercooler hoses for any cracks or loose connections. Also, the boost pressure sensor itself could be faulty. A faulty sensor can cause the engine to misread the boost pressure, leading to the symptoms you're describing. Has the engine light been checked for error codes?

Boost leak, huh? What exactly is a boost leak, and how would that cause all these problems? And where exactly is the boost pressure sensor located? I'm not very familiar with the turbo system.

A boost leak is when pressurized air escapes from the turbo system before it reaches the engine. This loss of pressure results in reduced power, poor fuel economy, and can cause the engine to run erratically. The whistling and hissing sounds are telltale signs. The boost pressure sensor is usually located on or near the intake manifold. Given the symptoms and your mileage, I'd strongly recommend taking it to a workshop to get it properly diagnosed. They can check the sensor and the entire turbo system for leaks.

Just confirming it was the boost pressure sensor. The mechanic found a defective cable and plug connection. They fixed it for 95€ and the car is running smoothly again. Thanks for your help!