V50 Air Intake Leak

Sounds like it. Have you checked for any cracks or loose connections in the air intake system? Especially around the intake hose. That hissing sound is a telltale sign.

I visually inspected the intake hose but didn't notice anything obvious. Is there a specific area of the hose that's more prone to leaking, or a better way to check?

They can be tricky to spot. Sometimes the cracks are small and on the underside. If you can't find it, a shop can do a smoke test to pinpoint the leak. But yeah, intake hose leaks are common on older cars, and that would explain all your symptoms. Probably worth taking it in.

Thank you for the advice. It turned out to be a leaking intake hose, as suspected. The repair cost 100€.