As mentioned, your pump is cavitating. Drilling the outlet fitting increases the maximum discharge flow rate although there is no saying just how much you increased it because controlled flow rate is a function of both the orifice size and spring rate behind the metal spool and with OEM pumps, spring rate can vary from pump to pump, even among pumps of the same type/model. Increasing the feed hose size and pressurizing the reservoir with a pressure cap up to 10-12 psi will help although if you made the flow rate too high, it might still whine due to the pump not being able to fill it's internal cavities completely.
I'm curious, does the whine start coming on strong at a specific engine speed or is it whining even at idle? Also, what size orbital do you have installed?