It's really very technical and hard to explain in laymen's terms but I'll give it a shot...
Its the patented Toyota starter wheel two stage system. The smart key main purpose is to let two small starter hamsters know your starting the car and that they need to start building momentum to get the, in your case, six larger drive hamsters going on their wheels. The wheels are attached by belts and cogs and such and there's a lot of precisely timed interaction involved... you really must keep in mind and Avalon is a big car and Japanese Hamsters have very short legs.
...I honestly have no clue what the sound is but Welcome to TN!