2016 toyota prius C two engine revving noise while driving

What's the speed of the car at those times, how many bars of battery charge, and grade of the road (uphill / flat / downhill )?

 

That used to happen to me on one particular route. Battery wasn’t quite full, but I would get the engine revving, like it was trying to burn off charge, but I was going slow enough that it could have just gone into ev mode.
Could never figure it out

 

The car is programmed to feel pretty much like other normal cars, which includes slowing a bit when your foot is off the go pedal, the way normal cars do. The designers decided you would think it felt weird if it just kept coasting along freely then, because normal cars don't.

Of course, the way normal cars would slow some with the pedal released was just a natural consequence of how they were built, while the Prius had to be programmed to feel the same way. If there is unfilled capacity in the battery, the car will generate that feeling by doing a little bit of regen braking: capturing some energy of motion from the car and storing it into the battery. If the battery is closer to full, the car will instead capture the energy of motion and use it to spin the engine, with the throttle closed and no fuel or spark. You can feel and hear the same combination of slowing and engine revving in a normal car by downshifting it.

 

A good OBDII scanner + an Android phone with the Hybrid Assistant program will help you take readings from the engine and hybrid systems while driving.
Hybrid Assistant: RPM Chart