I've had my GT for about 3 weeks now and love everything about it - except when I open one or both back windows for the pup. Once I hit 40mph a vibration starts and it literally sounds like a helicopter is landing on my roof! It is the craziest thing and gives me an instant headache. My daughter sat in the backseat to try and troubleshoot it today and she thinks it might be coming from the back doors. When the windows are closed, the car is so quiet and smooth. Any ideas?