Once you've driven the car a fair amount, basically when the transfer case (Differential) is warmed up, the car will start making these cow noises. People say it sounds like a dying cow but it just sounds like a cow mooing. Noise will start around 18-25 mph once you let go of the throttle. Around 20-25mph it'll make the noise once you let go of the throttle. Decelerating under 20-17mph, it'll make the noise again. I've heard it around 38-40Mph also. Toyota has a tsb for this problem and it only affects Limited and Adventure trims with the AWD torque vectoring. But if you don't have that you shouldn't have a problem at all. Toyota stopped replacing the transfer cases since it's still making the same noise and in the works of updating the transfer case. Should be coming out around as of June to July of 2019. Only way to stop the noise for the time being is to put it on sport mode.

While coming to a stop, I was hearing a "clunk" noise at 18-20 MPH coming from the foot-well area that didn't correlate to a transmission shift. I found it correlated to the new AWD disconnect system re-engaging the rear wheels by setting my center gauge cluster to display the real-time AWD graphic. I would also hear a slight clunk when accelerating when the animation showed power being cut to the rear wheels to save fuel. Over the past week, the clunk while accelerating has developed into a grinding noise.

I've had a Toyota tech at a local dealership drive along with me. They had no idea that the RAV4 had such tech, nor could anyone in the service department explain how it worked, but assured me it sounded "normal" which doesn't inspire confidence. The car didn't make this noise until I hit around 1,000 Miles. Taking it to another dealership after the first one accused me of wasting their time and that I shouldn't worry because the car is brand new.

Update from Apr 21, 2019: After 3 service departments, the third finally diagnosed it as a bad transfer case. Many others on forums have mentioned the same thing. I had the part replaced, but the new one is starting to act up again after 2,000 more miles. Apparently Toyota is aware of the problem and is redesigning the transfer case in some way.