V50 Diesel CEL Whistling Hissing

Whoa, sounds rough! That whistling/hissing combo screams boost leak to me. The actuator valve failing on these is pretty common, especially with those symptoms. Have you checked all your vacuum lines for cracks or loose connections? What about the intercooler hoses? A split hose could definitely cause all that drama. Is the turbo wastegate arm moving freely? Start with the simple stuff before diving into the deeper end, ya know?

Good shout on the vacuum lines and intercooler hoses, I'll give them a proper once-over. The wastegate arm seems okay, but I haven't put a ton of force on it. Is there a reliable way to test the actuator valve itself, like with a multimeter or something? I'd rather not just throw parts at it blindly, but that hissing is driving me nuts!

Yeah, testing the actuator directly is tricky without the right gear. Honestly, with those symptoms and mileage, and your suspicion of the actuator valve, I'd bet money that's your culprit. The electrical bypass valves on those are notorious for giving up the ghost. You could try rigging up a vacuum pump to manually actuate it and see if it holds pressure, but honestly, save yourself the headache and just bite the bullet. A new one shouldn't break the bank, but be warned, getting to it can be a PITA. If you're not comfortable with that level of fiddling, I'd just take it to a decent workshop.

You were spot on! After checking the hoses (which seemed fine), I caved and took it to a workshop. Turns out the electrical bypass valve on the boost pressure actuator was indeed kaput. 250€ later, and it's running like a champ again. Thanks for confirming my suspicions and saving me from endless fiddling! I was about to tear my hair out, seriously.