Hitting noise coming from back seat area, when start the car. 23 prius

I wonder if you may have the rear seats folded down, any mods to the below deck foam, not using the luggage screen, etc.
Anything that might enhance sound transmission.

 

My 24 LE does exactly the same thing since day one. Now it has 27K miles. I assumed it was normal...Let me know if it turns out to be something bad. Thanks.

 

That's part of the emission control system and Federally mandated warranty for 8 years or 100K miles; longer if your car is registered in a CARB state.

I'd look at that TSB to see if they messed with that unit on the recall. Make sure this is properly and specifically documented; so if that part fails just outside warranty; you may be able to get it repaired for free.

Good Luck......