Same issue here. I've also visited my dealer, and was told that it's a normal characteristic of the M6, and " It's a muscle car, it's supposed to make a little grind noise"

..... I also swapped out the tranny fluid for Mobil1 Syn. ATF with no change....
I still have 4 years on my warrantee.... We shale see how "normal" it's going to be later on ....