Hi,
the OnStar microphone is attached to the side of the Overhead Console. Unscrew that and disconnect the microphone connector near the front edge. See, if this makes a difference and follow up accordingly, and do plug it back in.
Let us know, how this ends up.
There are fans in places you may not know about, so be nice to the dealer. They certainly have more experience with this than you or me.
Take care, Eggert
