Talisman wastegate valve issue

Sounds like a boost pressure issue. Have you checked your MAP sensor? Turbo lag, whistling and hissing is often a sign of that, causing the check engine light and reduced power. Could also be a wastegate issue, which controls the turbo's pressure.

Wastegate? What's that exactly? And how would I check it?

The wastegate is a valve that regulates the boost pressure from your turbo. If it's faulty, it can cause overboost or underboost, leading to your symptoms. I'd suggest taking it to a trusted workshop to diagnose, as it will be hard to check it yourself. A defect in the wastegate valve can cause such issues, and ignoring it may damage your engine.

Thanks for the advice! I took it to the shop, and you were right – the wastegate valve was the culprit. Cost me 860€ to fix. Runs like new now!