Volvo S80 Turbo Lag Whistling Noise

Sounds familiar. I've got an S80, a bit newer. Had similar symptoms - whistling noise, check engine light, and the rest. I first thought it was the throttle body but that was fine. Have you checked your boost control system?

Boost control system? What part did you look at? I'm not super savvy with the tech, so any pointers would be awesome.

For me, it turned out to be a defect in the wastegate valve. It's best to take it to a workshop; diagnosing it precisely can be tricky. No point messing around with it if you're not sure; you could end up with a hefty bill. Getting it fixed cost me around 860€.

Taking it into the shop. Sounds like you’ve been exactly where I am. I appreciate the advice; knowing it's likely the wastegate makes it way less stressful. I hope they don't find anything else. It turns out, the bill was exactly 860€, and indeed, the problem disappeared. Thanks again!