At first i thought it was a transmission issue but have since learned that Prius transmission is built different from other cars . It's a gravelly, bubbley sound and it lags a bit when vehicle is going from a first gear speed to a 2nd gear speed . Also can happen while idling in drive... wierd. I'm into doing my own repairs if possible! Thank you for any clues to problem
How many miles are on your car? Have you checked your fluid levels to make sure they are all full? Oil, coolant......