If it ain't a nail, pebble, wheel bolt or fender liner issue, then maybe your tire has a 'flat spot' due to wheel lock-up under a hard braking manoeuvre.
F1 drivers try to avoid lock-up because the 'flat spot' rattles/shakes them up.
"Sometimes a lead-footed driver may accelerate to a level where the force is more than the grip can cope with, and will get wheelspin. The same may be applied to braking, but in this case rather than the track being stationary and the wheel accelerating, it is the opposite. The driver tries to slow the wheel whilst the track is still moving. If the amount he tries to slow by is more than the grip allows, the wheel will slide, the rubber will be worn away by the abrasive track surface - leaving plumes of smoke - and the driver will be left with Eddie Irvine locking up during the 1998 European GPa ‘flat spot’. This is exactly as is says - a flat part on the normally round tyre, and will cause the car to have vibrational and handling problems. To prevent this occurring, systems such as traction control (now banned) and anti-lock braking electronically control the amount of force input by the driver, and will not allow any more than that which will take the tyre to the limit of adhesion."
(
http://www.atlasf1.com/99/dec29/gray.html)
