So I've had my 2006 Cobalt LS for about 4 years now and it has treated me great up until about a year ago. When I'm doing highway speeds (65 - 80 mph), my front passenger tire (I think) starts to vibrate as if it were flat or had a rock or something in it. The vibrations increase when I hit my brakes, and by increase I mean cause the whole car to shake. I've taken it to mechanics and they've all told me they can't find anything wrong. I was just wondering if anyone else has had problems like these and if anyone has any idea how to fix it???