disclosed is an apparatus for vibrating a downhole drill string operable to have a drilling fluid pumped therethrough . the apparatus comprises a tubular body securable to the drill string and having a central bore therethrough , a valve in the tubular body for venting the drilling fluid out of the drill string and a valve actuator for cyclically opening and closing the valve . the method comprises pumping a drilling fluid down the drill string and cyclically venting the drilling fluid through the valve so as to cyclically reduce the pressure of the drilling fluid in the drill string . the valve can comprise a tubular body port and a corresponding rotor port selectably alignable with the tubular body port as the rotor rotates within the central bore . the valve actuator can comprise at least one vane on the rotor for rotating the rotor as the drilling fluid flows therepast .