A British thermal unit (Btu) is a standard unit of energy that is used in the United States and sometimes in the U.K.
There is another defect in tires that can cause a vibration called loaded road force variance. The tires and pass a visual inspection, a radial and lateral runout test, be perfectly balanced, but still cause a vibration. This is due to the internal defects in the tire.
When the frequency and magnitude vary with time, the vibration is random. Buffet is a form of vibration usually caused by aerodynamic excitation. It usually is random and associated with separated airflow. For example, buffet may be felt during the extension of speed brakes or during air turbulence.