It may not be the problem, but is worth looking into. But make sure your tires are running the same psi in the front as the backs. I had a similar sound/feel when I had a slow leak on one of my front tires, and everytime I equalled out the psi, it went away. If it isn't the tires, you may have too much "rake?" (rear much higher than the fronts?). It may be something as simple as that... Just a suggestion. Hope you figure it out though.