I've noticed the front wheel "feeling funny" and thought it's low air pressure. But after filling the tire it still felt a bit unusual. Took the hands away from the handle bar at 70km/h and it slowly started wobbling from left to right, quickly accelerating and getting stronger so I had to grab the bars again. It's going straight, doesn't pull to the side, but starts shaking. Never had an accident; tires are in good shape. It doesn't feel too bad when I'm riding normally but of course I want to know what the cause is. Mileage is almost 30,000km, bike is five years old. Any ideas what may cause the front end to wobble?