I recently bought a 2005 Cobalt LS Sedan.
I found that when driving on the highway and both rear windows are rolled down a very strange vibration is felt. It's kind of like a "sonic airwave disruption" inside the car, for lack of a better description! It will actually hurt your ears!
Roll the back windows up and the vibration is gone.
BTW, This isn’t a problem with the front windows down.

Now with that said, I have the oem tires - Conti's Contact AS.
Well actually I have 3 of them and an odd replacement tire, the brand is Lexington.
The replacement is thecorrect size and very similar tread pattern but the shoulder has a more square edge compared to the Conti’s rounded edge.
Anyone think this is the source of the strange vibration?
And why is it only noticeable when the rear windows are down?
I’m thinking about putting that tire on the front and see what happens.

And one last thing on the tires, I've read on this forum and others that the oem Conti's aren't the greatest tire for the Cobalt.
Any recommendations for a good all-season performance tire that can handle snow and ice well?

 

Changing the tires won't help with the noise you get. It happens on my trailblazer as well.....just when the rear windows are down driving on the highway. The "vibration" you get is the wind coming in and out of the car at just the right speed with the right amount of volume. When it happens to me i just roll one of the windows up a little bit and it usually cures the noise.

 

I have the same problem withmy 2006 Cobalt. I think it is an aero-dynamic issue and my only advise....dont drive with the rear windows down. I read an artical that says it is actually more fuel efficent to run a car with the AC on than with the windows downs because the drag with downed windows cause results in worse fuel economy than the AC running.

 

old question, but yes it's due to the air pressure.... Crack your front window and it will go away..