Noise is a sound wave of higher intensity. As any other waves, its intensity decreases as when it encounters obstacles in its path or when interference happens. The trees(generally the leaves) which act as obstacles in the path of the wave, dampens the wave. Its intensity decreases and hence the noise is reduced.
Apart from this, wind passing through the leaves produces a pleasant sound which suppresses the noise created from vehicles on the road.
(Note: Noise is the unpleasant sound of high intensity which generally hurts the ear.)