Seat Tarraco diesel engine issue

That sounds awfully familiar! I have a Seat Tarraco as well. I had almost identical symptoms a while back: check engine light, power loss, plus that awful whistling sound. Have you checked the air temperature sensor and its connections? It's like when a wire comes loose on a Christmas tree light – everything goes haywire! Is the temperature fluctuation plausible with airflow sensor data?

Air temperature sensor cable connection, you say? Where exactly is that located, and is it something I could check myself without making things worse?

Yes, that's what it sounds like. While you could try fiddling with the connection, and it might temporarily fix it, I'd recommend getting it properly diagnosed at a workshop. Those sensors are pretty sensitive, and you don't want to cause more damage. Could be as simple as a loose wire, but better safe than sorry. I bit the bullet and took mine in. They confirmed the faulty connection, cleaned everything up and secured it properly. Problem solved.

Took it to the workshop and they confirmed a faulty connection to the air temperature sensor. 95€ later and it's all sorted. Thanks for the help!