Noise within a hydraulic system is a high-intensity ambient energy source that can be harvested to enable wireless sensor nodes. The noise is typically due to deterministic sources, generally caused by pumps and actuators, and has dominant frequency components around hundreds of Hertz. Hydraulic pressure energy harvesters (HPEH) are centimeter-sized devices that convert the noise into electricity via coupling of the fluid to piezoelectric materials. HPEH devices produce milliwatt level power, which is sufficient for low-energy sensor nodes. A common device used for amplifying or absorbing acoustic energy is a Helmholtz resonator (HR). Incorporation of a HR into an HPEH has been predicted to increase the HPEH power response by up to 7 dB. The properties of hydraulic oil cause HPEH-sized HR to resonate well above the dominant frequencies. Added compliance into the resonator allows the resonance to be tuned closer to the dominant frequency within the hydraulic system. A prototype HPEH with an integral Helmholtz resonator was developed and tested. The results are also compared to an electromechanical model developed for HPEH-HR devices.