2011 qx56 auto 4wd feels like it's trying to engage

[mhead55]

My QX56 has only Auto doesn't go just into two-wheel drive and here lately when I let off the gas or when trying to turn it feels like a bumping, like it's trying to engage the four-wheel drive it's like pretend there's a button that you can push to hold down to engaged four wheel drive it's like somebody's tapping the button over and over. When you put it four wheel drive it doesn't do it anymore anybody have any ideas