Gen 3 Engine Noise??

Blown head gasket. It tends to allow coolant to leak into combustion chambers overnight, create compression havoc at morning start up. If you stick with it be sure to clean EGR system and intake manifold, and repeat cleaning at least every 50k miles.

see my signature for some head gasket and EGR links, plus last link is full engine section from repair manual. (On a phone turn it landscape to see signatures.)

 

How many miles on the car? Has the temp been hot during the day and
then cooler at night? It might be condensation, it's happend a lot with the Prius.

Check the coolant tank, is it low? If it is, you could have a head gasket failure.
DO NOT PUT ANY SO CALLED HEAD GASKET SEALER IN THE COOLING SYSTEM!
It will just cause you huge headaches and cost you a LOT more money and work
when you replace the head gasket.

If it is the head gasket, change it now. You caught it before it causes worst issues.

 

Check the coolant level, Prius owner usually never top up the coolant since it is practically sealed until the overflow reservoir reaching certain pressure and the valve opened.
In California, You have Head Gasket Master, a Mobile mechanics who can do it on your parking lot for quick, cheap, and right. Gasket masters - YouTube
If your car already consumes a lot of oil, say 1 quarts every 1000 miles Autobahn/Interstate drive, just get a used 2015 engine instead. It solved both the head gasket and oil consumptions. Otherwise, do he head gasket repair properly but check if it is not an stuck open EGR valve.

It happened to me 4 years ago when I open the black plastic cap on the EGR. I install the screw-in rotor not all way down before installing the black magnetic cap in. That cause the EGR valve open at zero position. It rattles and shakes violently. I stopped and fixed it right away.I was just curious if the EGR is not clogged and move freely on the plunger.