Hello, I have a 04 5.9 4x4. I have recently started using 4x4 as it has started to snow and ice in Indiana. I noticed that after shifting into 4x4, my truck makes a knocking noise. The knocking is not very loud when accelerating, but when I let off the petal and begin to slow down the knocking gets louder. The 4x4 does work it just knocks. Should I be concerned? Is this normal?