I am hoping this is normal Subaru behavior (this is my first Subaru or any AWD car). I am guessing it has to do with the AWD figuring out what is going on and what to do.
04 OBW 6cyl Auto. When starting from a red light or stop sign, and making a left turn or right turn (the steering wheel is already cranked left or right), I hear a sound for about 1/2 second coming from the left (or right) rear. It sounds like a sort of slight rumbling sound, and you can feel a very slight vibration in conjunction with the sound. Almost as if there were noisy snow tires on the car. You wouldn't hear the sound (or be aware of the vibration) if the radio is on.