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?

 

It's called wind buffeting. Crack one of the front windows open as well, and it should go away.

 

dlsea, is this your 1st ever vehicle?

You will have this in practically every single vehicle. As mentioned above, its called wind buffering and is to be expected.

 

Lol, I do keep my cars a long time! I'm coming off of a 2005 Subaru Outback, which was apparently so un-aerodynamic that it didn't have this issue. Now that I have a name for it (wind buffering, wind throb or Helmholtz Resonance), I'll figure it out. I'm so glad nothing's wrong with the car!

 

Just wanted to mention - regarding the original purpose of having the back windows open for a dog - one of my relatives is a veterinarian, and he strongly advises against letting dogs stick out their faces into the wind in a moving car. Anything, even as small as a grain of sand, that hits their eyes at speed can potentially cause serious damage to their vision. He sees a lot of cases of this.

 

I believe it. Our girl is harnessed in and always wears her eye protection!

 

The problem also exists with front windows. The best combination is having the opposite window open. Front left/back right or front right/back left.

I was not sure if it was only a Mazda thing since I didn’t notice it on my wife’s Subarus only my Mazdas.

 

Nothing wrong with the car...this is common on many, especially on SUV's.
As others have suggested, opening an opposing window on the front will mitigate/eliminate the sound. You may also find that adding front window deflectors such as those from WeatherTech will further minimize, also will help cut down any cold drafts (that probably isn't an issue in San Diego, but is here in NJ!).