The hydraulic buck converter (HBC) is a novel high-bandwidth and energy-efficient device which can adjust or control flow and pressure by a means that does not rely on throttling the flow and dissipation of power. However, the nature of a HBC can cause severe fluid-borne noise (FBN), which is the unsteady pressure or flow in the fluid-filled hydraulic circuit. This is due to the operation nature of a high-speed switching valve of the device. The FBN creates fluctuating forces on the pipes which lead to system structure-borne noise that develops air-borne noise reaching to 85dB. Thus there is a need for an effective method that does not impair the system performance and efficiency to reduce the FBN. This paper describes the first investigation of an active controller for FBN cancellation in a HBC based on in-series and by-pass structures. The dynamics and the noise problem of the HBC are investigated using the analytical models. A piezoelectrically actuated hydraulic valve with a fast response and high force is applied as the adaptive FBN attenuator. The performance and robustness of the designed noise controller were studied with different operating conditions of a HBC. Simulated and experimental results show that excellent noise cancellation (30dB) was achieved. The proposed active attenuator is a very promising solution for FBN attenuation in modern digital hydraulic systems which promise high energy efficiency but suffer severe noise or vibration problems in practice.