ok back to what the op asked... the same thing happens on my car and so I've been trying to find an answer. GM actually posted a bulletin about it, and I'll paraphrase.
When shifting between park and reverse, park and drive, or drive and reverse, a clunk noise is the result of a buildup of lash (freeplay) in the driveline. Owners may comment the clunk noise happens when the accelerator is quickly depressed and then released. In service dealers are discouraged from attempting to repair driveline clunk conditions for the following reasons:
1. comments of driveline clink are almost never the result of one individual component with excessive lash, but rather the result of the added affect of lash present in all the driveline components.
2. Because all of the components in the driveline have a certain amount of lash by design, changing driveline components may not result in a satisfactory lash reduction.
3. While some owners may find the clunk noise objectionable, this will not adversely affect durability or performance.
The sad part about the whole bulletin is it just simply says "poor engineering." Hope this answers your question though!
