Have a 2011 Forester for three weeks, but only drove with a rear window down a few days ago. When windows are up, no vibrations or noises and is a great ride. When the rear window goes down, well it's more than annoying, it's deafening. A very low frequency vibration begins at 50 mph and intensifies with more speed. I hoped it was just a bad tire, but the service manager at the dealership said "It sounds like normal wind buffetting to me. It's normal." Can this be true? He said the noise can be controlled by opening other windows some, so I lowered both rear windows and the noise stayed the same. I've never encountered this in four decades of driving, have any of you?