Gen 4 odd engine noise like sloshing water(not knocking of previous threads)

I started my Gen 4 Prius two yesterday and got out to adjust a bolt on the rear license plate and noticed a strange sound as the ICE in high throttle began the warm-up cycle- a sound like water sloshing around in the cat converter (or somewhere upstream?). Other new Gen 4 owners please check this and see if you hear this on startup. It is not regular enough a sound to be engine knocking, but literally like water churning. Not loud enough to hear from anywhere but right near the lower rear of the car. Anyone hear this on their new 2016? My Gen 3 never had this sound.