Unusual Engine Whir on Startup

Hi everyone!
I have an 05 Prius in California that my mother is currently driving. It started making a new noise on startup a couple months ago, but stops after the car starts moving. It's a whirring sound that reminds me of a motorcycle.
Car currently has about 350,000, I don't know for absolutely certain because the odometer is one of Those.
I wish I could upload the audio for it, but I don't know what the correct file type is and it won't let me.