Annoying high pitch hissing sound when accelerate / deccelerate

i usually grab a length of hose and use it like a stethoscope around the engine to pinpoint sounds more accurately - to me sounded louder from your short video when you were near the electronics on the right side of the engine bay as you face it.

 

That reminds me almost exactly of one of my worst panicked moments as a teenager, when I had reassembled an engine and it made a horrible whistle just like that, and I assumed I had achieved some kind of epic fail.

I took it very sheepishly to the old-timer mechanic up the road, and he listened, and grinned, and asked me if I had re-used the gasket under the carburetor, which I had.

The gasket was a little flattened by reuse, and with the vacuum inside the manifold, it was admitting a tiny amount of air, and being a clarinet reed.

A Prius doesn't have a carburetor, but it does have a throttle body, with the same sort of gasket under it. There are also some other gasketed connections in the vicinity of the intake manifold.

What nearly clinches it for me is in the very last seconds of your video, when the whistle drops out for just a moment, when you suddenly rev the engine, and then the whistle returns.

Intake manifold vacuum becomes very weak for a moment when suddenly revving the engine, and then returns.

 

+1 for some kind of vacuum leak.

Posted via the PriusChat mobile app.

 

Yup, that's a big vacuum leak....

A spray bottle of water would find it pretty quick

 

Doesn't have to be a big leak at all, as I learned on that occasion. My old gasket looked just fine too; the key thing is every time it's installed, it gets mashed down a little flatter in some key spots. When just a small amount of air being pulled over it can make the thin spot flap like a musical instrument reed ... there's your surprisingly loud tone.

A big vacuum leak would also be giving you rough running, P0171, etc. This is a small but loud one.