Buzzing noise when shifted from drive to park,from rear.
Isn’t this car still under warranty? That would mean you should take it to the dealer for this.
My guess would be a vibration in the exhaust. It may be nothing but as Mustangman said it should be under warranty.
Since the buzz occurs in park, put it in park, and if you hear the buzz walk to the back of the car and see if you can figure out what it is that’s buzzing. You could use a length of old garden hose as a noise-locating stethoscope. It could be something inside the car, the spare tire area, etc.
Another idea, vehicles test their gadgets when the driver does something such as putting the transmission in park, so it could be something like that. The ABS system and the evap system are two such tests that might be related to the sound. Ask your dealership to test this theory out on another RAV4 they have on their lot maybe it is just normal sound for this car.