First off, hi, just joined the site tonight. I’ve had my 07 convertible for 2 weeks, purchased as certified used, 34,000 miles. I have mine in the shop right now for this same problem. Service guy says it’s the exhaust, something related to a small hole he said that causes a whistle. He also told me that he’s got a service bulletin from GM acknowledging it but it says not to fix it. I’m in the process of going through the service manager and then I’ll go to the owner if necessary. I knew the car had this squeal when I test drove it, mentioned it to them and they said no problem they would fix it when I brought the car back for a seat belt guide that was broken. I’ll get to the bottom of it one way or another or I’ll tell them to take the car back. There is absolutely no way I or anyone else in my opinion would buy a car knowing it had this squeal and could not be fixed. I think I remember reading on Edmunds that other have had to have the exhaust changed by the dealer to fix it.i'm prety sure Jetmech had the same problem and found he had a small hole in his flex pipe and he fixed it. Maybe he'll see this post and chime in. I'll see if I could find his post.