I drive a 97 GMC Sierra Z71 Off Road and every time I put it into 4 wheel low, I hear a really loud "clunk"
I am always stopped and in netural, as required for 4 low of course and it is always there. Makes same noise taking out of 4 low as going in. Anyone have any idea what this noise is or if it is normal???

I am always stopped and in netural, as required for 4 low of course and it is always there. Makes same noise taking out of 4 low as going in. Anyone have any idea what this noise is or if it is normal???