My sonata has started having issues on the highway at speeds ~65-75 mph. It will start vibrating, a little at first and gradually getting worse. As this happens my brake pedal seems to be more firm and touchy rather than soft and needing more pressure to stop. The vibration can be felt in the steering wheel and the floor as I drive. This has happened twice so far, with a gap of about a week in between incidents. What should I get checked out?
What are you doing with your brake pedal on the highway at 65-75? If you’re driving attentively, you should rarely need it at those speeds. That said, is this vibration associated with braking only? Or perhaps with accelerating only? Or perhaps just with the speed no matter what else you are doing? The answers point in different directions.
If the steering wheel is shaking at 65-75mph with your foot OFF the brake pedal, you quite possibly need to have your rims balanced
When the car began to vibrate, I exited the interstate and noticed the brake getting stiff. The vibration was happening regardless of acceleration or braking. As far as I could tell it began when I reached the higher speed of the interstate. There are no problems when I drive on surface streets.
Feel your wheel hubs after a drive. A hot one means brake problems.
If it does it braking or not, more than likely a tire is out of balance or bent rim. I have had tires separate internally on me and that causes a shimmy. They had to lift the tire balancer while it was spinning to see the tread shifting back and forth.