Sound When Powering On

The sound you hear when you power up cold is the pump circulating warm water from the thermos bottle that helps heat up the engine to operating temperature. I'm not sure what the deep thud sound is when the engine is warmed up.

 

might be the brake barking sound some older brake master cylinders make. not an issue if the noise doesn't bother you.

 

My 2008 Prius (UK) makes a delightfully nerdy thrumming noise first thing in the morning, when I depress the brake pedal before initiating "primary boot up sequence". I recognise this sound as coming from the brake's servo assistance pump. This is perfectly normal for this model - or so I am told.

I might sometimes hear the same noise when crawling in traffic, but always only when depressing the brake pedal. And sometimes, I hear nothing at all, especially after a long smooth road run.

Crawling to a slow halt when the brake discs are cold, occasionally, I'll hear a gronk, as the front brake pads grip the discs and force the wheel to a halt. It usually depends on how hard I depress the brake pedal, and when, after that moment when regenerative braking sequence switches over to hydraulics, to bring you to a 'physical' halt. Could this be the thud of which you speak?

Once stopped and walking away, or otherwise unloading stuff unhurriedly from the cargo compartment to the rear of the vehicle, I'll often hear the same thrumming noise, and indeed a raft of other mechanical gasps and sighs from the propulsion compartment. This is again, nothing to worry about. When I hear this, I am reminded of THAT scene of the Millenium Falcon (in Starwars: The Empire Strikes Back ) immediately after it had landed on THAT platform on the Cloud City of Bespin, and was venting gasses from around it's landing struts, as it was progressively powering down ancillary propulsion and life support systems. Your Prius II spaceship does something quite similar too!