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.

 

The noise sounds like the normal operation of the AC compressor. Try turning the climate controls to OFF and see if it goes away.
The code P0420 - catalyst efficiency, does not automatically mean a bad catalytic. Have your front A/F sensor and rear HO2S sensor checked for proper function when hot. It is more likely the front A/F sensor is wearing out and close to setting a fault on its own. Catalyst efficiency is determined by comparing the signals from the two sensors over time. I've replaced many Gen II catalytic converters due to theft, but not one due to the catalytic itself failing.
The combo meter. When the headlights are off, if the dash dimmer is turned all the way down, the instruments do not show at all. Try turning the dimmer to high (the thumbwheel to the left of the steering column). If they are still dark, they can be repaired. There is a component on the PC board that fails. A good speedometer shop should do the job for much less than a replacement combo meter.
The Traction (hybrid) Battery is probable fine. If you have doubts, have someone with the Toyota Techstream scan software check it out. Most generic scan tools can't access all the HV battery parameters. The 12v AUX battery is more likely to give you trouble. Have it checked. If it is marginal and/or over 3-4 years old, replace it.

And most important, Enjoy your new purchase!