The paper presents experimental and theoretical investigations on a shaft mounted gerotor lubricating pump aimed at reducing radiated noise at high engine speed. Effects of noise generation identified as main sources are the fluid borne noise (FBN) that originates in unsteady flow and related pressure fluctuations and structure borne noise (SBN) as a result of pressure transients occurring internally, which cause vibrations of the pump case. To clarify the onset of large delivery pressure fluctuations detected at high pump speed (in excess of 4000 rpm), and validate simulation results (AMESim environment), experimental and theoretical studies have been performed. The basic hypothesis is that incomplete chamber filling at high speed (clearly demonstrated analysing the steady state flow-pressure characteristics and the volumetric efficiency), gives rise, when a chamber starts being connected with delivery, to an intense and rapid reverse flow towards the chamber, this entailing an expansion wave followed, at complete filling, by a pressure peak.To alleviate the problem a number of interventions are critically scrutinized experimentally and theoretically following two approaches: optimal design of silencing grooves to smooth reverse flow into variable volume chambers; provisions to arrive at complete chambers filling upon leaving the pump suction side; Within the latter approach two gearings are contrasted at equal pump case. The basic design with an odd number of variable volume chambers and the alternate gearing with even chambers. A real-time frequency analizer allows monoaxial intensimetry analysis of air-borne noise while, elaborating on the instantaneous pressure ripple at pump delivery, fluid borne noise is also detected; both informations being of prime relevance as part of a pump design and development programme.