Originally Posted by dparm
Exactly. The center differential attempts to equalize the rotational speed of your four wheels, and struggles to compensate at slow speeds when at extreme turning angle. It's just quattro; sometimes you may even hear a noise like a foghorn in this type of scenario. The problem is lessened when you speed up. Moral of the story, avoid these situations, as it places additional wear on the center diff. Remember that you can't tow a quattro with two tires on the ground for the same reason: when all four wheels are not rotating at the same rate, quattro torsen diff suffers greatly, and will
burn out if hitch towed with two tires on the ground. The experience you are having is similar to this, to a lesser scale of course.
When you are turning slowly at full turn, the difference in rotational speed of your other wheels compared to the inner wheels, at slow speed, is much greater proportionally, to the same full turn at higher speed. Thus, you'll see this during slow-speed full turns versus cruising speed full turns (if you can manage that, like doing spinnies or drifting at speed, you will not hear the center differential groaning; you risk damaging your steering pump instead if you hold at full turn for too long)