Toyota Echo 2005 automatic- Just bought it- I backed out of a parking spot on a hill and it made this strange whirling noise...What could it be?

No, I bought it from a private party- a family that was moving out of state with 2 kids- The young woman was clueless about the car- (she has a new 6 wk old baby and a 4 yr old- so she's busy with her kids.)No records or owners manual... I'm going to take it to a mechanic and see what he thinks. This is the newest car I've ever owned- perhaps it's some computerized sound?

You stated it made the noise when on a hill reversing, but not on flat ground. Perhaps the noise is coming from the transmission reverse gear when under a strained load.