Rattling noise Comming from Under dash acceleration issue

I don't know what the rattle is but the being stuck in third gear is normal. Fluid doesn't have anything to do with it. The trans in a Prius is more like a manual because the fluid just lubricates but it is not used to pump in a converter. It's all electronic so the computer keeps the revs at whatever is best for the current situation. It is high when accelerating but is only around 2,100 rpm at 70 which is what my 02 vette with a 6 spd runs at. Also if your battery is low at the time it will be higher then usual because most the torque comes from the electric motor.

 

I would speculate the rattle you hear is the brake actuator which is pressurizing the brake system. Mine started running excessively, had to change it, and now I don't hear anything but a click or two when the brake is depressed. Bleeding the brakes helped mine for several months, something to consider, but my clicking came back (up to 20 sec's at a time), so I changed it.

 

Yes its the brake vacuum pump (actuator).
You can replicate that sound by just pressing on the brake pedal firmly 3 times with the car off. Is that the sound?
Sounds just like a rattle snake.
And the over reving is normal. Its just the engine and planetary gears doing there thing. Its not actually over reving it just sounds that way.