I get a sound like that too, but it isn't the battery. It is normal though - there's a sound to the regenerative braking that's quieter, but you're right there's a separate sound that happens on occasion that's almost like a "I'm switching modes" sound coming from the Prius. Nothing to worry about, just it keeping everything in check. You don't so much drive the Prius as ask it to do things, and it meticulously plots a low fuel usage way to accomplish that. Several gurgles and buzzes later, and you're on your way.

Prius 2006, bought September 14: