Etios diesel engine power loss

Sounds like you might have an intake manifold issue. The hissing noise especially points to that – it's likely an air leak. Reduced power and rough running would also fit. Does the engine light come on? The intake manifold is what distributes the air evenly to the cylinders. If the flaps inside aren't working correctly, and they control the amount of air going in, it can mess with the air/fuel mixture and cause those symptoms. Have you checked for any error codes?

Thanks for the info. Yes, the engine light is on. I haven't checked for error codes myself. What exactly do these flaps do, and how would a faulty switch cause all this?

Okay, the engine light confirms something's definitely up. Those flaps I mentioned are part of the intake manifold control system. They adjust the airflow based on engine speed and load. A defective switch controlling them can cause them to stick or not open/close properly. This messes up the air-fuel ratio, leading to reduced power, rough running, and the hissing sound if there's a vacuum leak. It's best to get it checked by a workshop. They'll read the error codes and diagnose the exact cause. Continuing to drive it with this issue could potentially damage the engine further.

Thanks, I took it to a workshop, and they confirmed it was the intake manifold flap control switch. Repair cost was 465€. Appreciate the help!