Loud noise from under the hood when engine turned off..

you're probably covered under your warranty take it to Toyota let them look at it tell them the signs let them fix it

do you have any lights that come on your dash when that happens?

 

Can you describe the noise? Maybe it would help to analyise the condition. Sometime the A/C will make hissing noise when the pressures in the system are equalizing after shut down.

 

Do you put the car in Park by pressing the Park button before shutting down the car using the Power button?

If not, then I've noticed that the car can make a noise when engaging the parking pawl before/while the car is shutting down.

 

Yes, but a lot of people bypass this process because the car will automatically put the vehicle in Park before shutting down. I've noticed that a message will flash briefly reminding you to put the vehicle in Park before shutting down on the MFD, or was it the Navigation screen?

 

The next time you power down your Prius, try pressing the Park button first. I suspect that that will stop the "thunk" noise you are hearing when you power down your Prius.

 

With a post lke this, I'm thinking that something out of the ordinary is happening. Evidently the OP's post was inquisitive and not intended to suggest something was wrong with the car. If indeed the noise is just the park mechanism, I would hope that's the case. After clocking 20,000 miles on the car, intuition would point towards "Normal".