If you choose to do this, make sure you do the maths following Ohm's law correctly. A potentiometer/variable resistor needs to be rated for enough power that will be dropped across it. Pumps draw more current than fans. You could have switchable resistors for 2 or 3 speeds instead. Either way, there's a small chance the pump will misbehave if it's current limited, but I don't think it should be an issue.Thirdly if needed , try header from different pc. If it is the PWM controlling of the pump that is causing the whine then research how to control the pump via molex, guess an old fashioned variable resistor in series could work.