S80 Diesel Reduced Engine Power

That sounds eerily familiar! I had almost the exact same symptoms with my 2014 Opel Insignia a couple of years ago: reduced power, rough idle, and a weird jerking feeling when I tried to accelerate. The hissing noise is a big clue, too. I didn't check the intake manifold control myself, but the mechanic mentioned something about airflow being all over the place. What kind of error codes are you getting, if any?

I'm getting a persistent P2015 code, which points directly to an intake manifold flap position sensor/switch issue. Did your Opel throw the same code, or something similar?

That P2015 code rings a bell. I remember the mechanic mentioning something about the intake manifold needing a complete overhaul. Seeing as you're getting that code and experiencing similar symptoms to what I went through, I'd say it's highly likely you're on the right track with your diagnosis. I'd recommend getting it checked out by a professional. It cost me about 500€ back then, but it fixed the issue completely.

Your advice and the comparison to your Opel experience solidified my suspicion. I took it to my local independent Volvo specialist. The repair cost 465€ but resolved the problem completely. Thank you again for your help!