Weird 2007 Prius intermittent whirring noise??

welcome!
sounds normal, is this your first hybrid?

 

does the noise repeat? If so, how often?
Is likely the brake pump turning on to pressurize the brake accumulator. If it cycles frequently while the brakes are not being used, it's indicative of the accumulator leaking.

 

So does this sound only occur while the engine is running? Idling means the engine running.

By the way, post five times to wake up the board.

 

The brake booster sound often occurs without the engine running. Test by carefully listening to the drivers side engine compartment when you first open the drivers door with a fob first thing in the morning. This will activate the brake booster for at least five seconds. That sound is normal and occurs even before you have a Ready state. Typically it will run occasionally when driving but is hard to hear over road noise or engine noise. Sometimes you can hear it at a stop light when the engine has cycled off.

The point is, learn the normal brake booster sound. It will occur even with the engine off or cycled off by the hybrid system. If you hear something different and only with the engine running, I would have it diagnosed.