Regulate DC-Pump throughput
Buck regulator is basically a PWM with a filter. If you use a PWM switch with a high temp low ESR cap. than can handle ripple current, you can provide DC control of speed easily to maybe 30% depending on stiction. 

Without knowing the details of your pump, my guess 1HP 72V 10Amp,  I assume you can reduce to  75% V+ to cut the flow and RPM in half but noise depends on brushes, gears and bearings.  DC is more efficient but noisier. YOu could consider sound proofing but not thermal insulating the motor.

Once you provide more accurate requirements, there are many creative ways to reduce the DC voltage. PWM , buck reg., add 500W Halogen Lamp in series etc etc.