XC70 diesel engine power loss

Sounds familiar! I had something similar happen with my XC70 a while back, though I can't recall the exact mileage. Was the hissing sound noticeable even at lower speeds, or just when you really put your foot down? Have you checked for any error codes? Mine threw a code related to the air intake system.

The hissing is definitely more pronounced at higher speeds, particularly when accelerating. I haven't scanned for error codes yet, but the symptoms you described sound pretty close to what I'm experiencing. Was it definitely the flap control in your case, and how was it diagnosed?

Yeah, after chasing a few different things and checking all hoses that hissing sound made me think it was a vacuum leak in the intake system, which I then traced back to the intake manifold flap. My local shop confirmed it with some diagnostic tools. It ended up being a faulty switch intake manifold control. I’d recommend getting it checked out by a professional to be sure, but the symptoms you're describing are very similar to what I went through. I got lucky that time.

Thanks for the info! Sounds like I'm on the right track, but I'll definitely get it checked by a mechanic. Appreciate the insight. They indeed confirmed the flap control was defective and replaced it. Total cost was 465€. Runs like a charm again!