Well I am concerned about your situation here cause I have an 07 model Grand Prix. I've never heard of the term "Frame Flex". I am under the impression just by the term itself that the frame of your car flexes, like moves or buckles and then returns to it's normal state, until acted upon again by a force, such as shifting from P to D as you have indicated. So the dealership has suggested they inject a substance into your frame, I am assuming to render it more ridgid to prevent this Frame Flex phenomena in the future. I would like to see a TSB or a recall on this from GM and I am sure you would too. Because if your car is flexing just from using the shift selector then you can expect your car to split in half from sustaining side impact in the case of a wreck. That said I would not submit to this dealers recomendations. If I had to guess I would say this poping noise you have experienced is from a bent or misaligned shift linkage or something associated with it. I hope you find the real cause and if you would let us all know.
