Prius bin bag sound

Plastic shopping bag? Have you looked under the car? It could be stuck anywhere on the bottom of the car, worse though if it is covering part of the radiator. On a different car we owned one melted onto the catalytic converter and the car smelled like a burning candle for a month.

 

Sorry, I should have just replied to your new thread here... Unable to delete my reply in the other thread.

It could be exhaust leaking out around your header/downpipe steel/ceramic donut where the catalytic connects due to a clogged catalytic creating too much backpressure and resistance to flow.

How is your performance / top end right now? Are you struggling to hit 85-90+mph on the highway with the gas pedal to the floor?

If it's like mine was it will start doing it more frequently and at lower rpm's yielding lower power output and more strange noises as the catalytic gets more clogged.

I ended up taking my cat off and scraping / pressure washing it to open it back up again.

Before:


After:

 

This probably isn't it, but it is easy, so ...

Check that the air filter is the correct type, is seated correctly, that the cover lid is fully locked down and the box itself is firmly in place. The most immediate result of stepping on the accelerator is that the throttle opens and the flow through all of that increases. if something is loose it could move around and it might produce the sound you hear. As long as you are in that neighborhood, check the air path upstream of the air filter.

Does it make the sound when you step on the gas and the car is in neutral? If so you can listen from the outside while somebody else steps on the gas.