I had a 2004 trailblazer that when in auto 4x4, would clunk when the rears slipped and the fronts engaged. Did that since it was new and was told that's normal. For that reason I always ran it in 4x4 Hi in the snow. My 15 Silverado in auto 4x4 is super smooth. No noise and can't really tell when it engages. It feels like an AWD vehicle. I have never put it in 4x4 Hi or Lo.