An agitator tool for introducing axial movement in a downhole tool of a drill string in a bore hole, where the agitator tool comprises: - a housing configured to be positioned in the bore hole, wherein the housing has a first open end connected to a second open end via at least an inner surface, wherein the housing is configured to guide at least a portion of a drilling fluid through the housing via the first and second open ends, - a drive unit,, configured to drive the agitator tool, wherein the drive unit is configured to be coupled to a first moveable element arranged inside the housing, - wherein the first element is configured to move in an axial direction relative to the housing for introducing axial movement in an external downhole tool coupled to the agitator tool when the drive unit drives the first element, - a second moveable element arranged inside the housing, wherein the second element is configured to be coupled to the drive unit and to rotate relative to the first element, wherein the first and second elements (5, 9) are arranged relative to a common centre axis, - the first element is coupled to the second element via mechanical coupling means for converting the rotating movement of the second element into the axial movement of the first element, - the coupling means comprises a first coupling element located on a first surface of the first element which is configured to engage a second coupling element located on a second and opposite facing surface of the second element, and wherein the first coupling element is configured to move along the second coupling element when the drive unit drives the second element, - wherein the second coupling element forms at least a first guiding section for forward axis movement of the first coupling element, where the first guiding section is connected to at least a second guiding section for backward axis movement of the first coupling element, and wherein the first and second sections form a guiding loop for the axial movement of the first coupling element, characterised in that - the first and second guiding sections has a modified sinus shaped groove where at least one of the guiding sections has a modified shape (43c, 43d) with at least a predetermined pitch and/or length, that differs from its symmetric shape for accelerating or de-accelerating the axial movement of the first coupling element. - a housing configured to be positioned in the bore hole, wherein the housing has a first open end connected to a second open end via at least an inner surface, wherein the housing is configured to guide at least a portion of a drilling fluid through the housing via the first and second open ends, - a drive unit,, configured to drive the agitator tool, wherein the drive unit is configured to be coupled to a first moveable element arranged inside the housing, - wherein the first element is configured to move in an axial direction relative to the housing for introducing axial movement in an external downhole tool coupled to the agitator tool when the drive unit drives the first element, - a second moveable element arranged inside the housing, wherein the second element is configured to be coupled to the drive unit and to rotate relative to the first element, wherein the first and second elements (5, 9) are arranged relative to a common centre axis, - the first element is coupled to the second element via mechanical coupling means for converting the rotating movement of the second element into the axial movement of the first element, - the coupling means comprises a first coupling element located on a first surface of the first element which is configured to engage a second coupling element located on a second and opposite facing surface of the second element, and wherein the first coupling element is configured to move along the second coupling element when the drive unit drives the second element, - wherein the second coupling element forms at least a first guiding section for forward axis movement of the first coupling element, where the first guiding section is connected to at least a second guiding section for backward axis movement of the first coupling element, and wherein the first and second sections form a guiding loop for the axial movement of the first coupling element, characterised in that - the first and second guiding sections has a modified sinus shaped groove where at least one of the guiding sections has a modified shape (43c, 43d) with at least a predetermined pitch and/or length, that differs from its symmetric shape for accelerating or de-accelerating the axial movement of the first coupling element.