Vauxhall Vectra losing power hissing noise

Sounds like the boost pressure actuator is indeed a prime suspect. The symptoms you describe, like reduced power and hissing, strongly point towards a leak or malfunction in the turbo system. The actuator controls the turbocharger's boost, and if it's faulty, the engine won't get the correct air-fuel mixture. This can lead to all sorts of problems, including the ones you are describing. What kind of error code shows up when the check engine light turns on?

Cheers, thanks for the quick reply! The error code? I haven't actually checked it yet, I should have I know! But if it IS the actuator, is that something I can fix myself, or am I looking at a garage job? And what exactly does the actuator do? Is it just a simple valve or is there more to it?

Right, get the error code read first, then clear it and check whether it comes back. That will confirm whether the sensor is really faulty, or whether the problem lies elsewhere. Actuator’s job is to control the boost pressure delivered by the turbo to the engine by managing the position of the turbocharger vanes. You COULD try to replace it yourself if you are confident with car mechanics and know your way around screwdrivers and wrenches, but considering all the symptoms, I'd recommend taking it to a workshop. They can properly diagnose the issue and replace the defective electrical bypass valve. You can buy the part yourself if you want to source a good deal, but let the workshop handle the installation for best performance. It's tricky to access and get right.

Thanks a million, your advice was spot on! Took it to the local garage, they confirmed it was indeed the boost pressure actuator. Got it replaced, cost me around 250€ all in. Car's running like a dream again! No more whistling, the power's back, and the check engine light is finally off! Much appreciated for steering me in the right direction. Saved me loads of time trying random fixes!