Barack Obama made a call for nonviolence in the aftermath of the Sean Bell verdict - infuriating the Rev. Al Sharpton, who accused the presidential candidate of trying to "grandstand in front of white people," sources told The Post. During what a source described as a "heated" phone call yesterday, Sharpton told Obama he was disappointed with the Illinois senator's words on Friday, when Obama said "resorting to violence to express displeasure" was "completely unacceptable and counterproductive."