I get an intermittent vibration when towing my ZX250 w a tandem trailer at highway speeds. It will start when I hit a sharp bump in the road. Looks like its when the 2nd axle hits it. Then if I hit another it will go away, if I don't hit another bump it will slowly stop. I've checked bearings no play, no grease problem, no heat( can even touch the rotor when I stop). Had guys follow and said its tracking fine. Cannot for the life of me figure this out, does anyone have any idea for me to chase?