My Firehawk(yes, I know it is a completely different car) did that as well, and of couse it had T tops. The point I'm making is, there may not be much that the dealer can do about it that you would want them to do. I fixed mine by installing subframe connectors( a popular fix for that problem on an F body), however, I worked at a Cadillac dealer and one of my customers had a similar issue with an early CTS we had to cut holes all throut the right rear of the car looking for broken spot welds!!! I don't know about you, but I wouldn't stand still for that.
Just make sure that you and the dealer can identify the noise as a problem before the work starts.