Clunk Not a Worry
Some 2012 and prior GM passenger cars and light trucks equipped with a 4L60-E, 4L65-E or 4-L70-E automatic transmission may exhibit a clunk noise that can be heard on a 2-3 upshift or a 3-2 downshift.
During a 2-3 upshift, the 2-4 band is released and the 3-4 clutch is applied. The timing of this shift can cause a momentary torque reversal of the output shaft which results in a clunk noise. This same torque reversal can also occur on a 3-2 downshift when the 3-4 clutch is released and the 2-4 band is applied. This condition may be worse on a 4WD vehicle due to the additional tolerances in the transfer case.
According to GM, this is a normal condition and no repairs should be attempted.