Operating the motor at reduced speed is self-explanatory; cooling air speed and friction are reduced, resulting in less noise created. That said, a given motor may experience increased vibration at characteristic frequencies, which can increase noise to a level greater than that produced at base (i.e. rated) speed. Most Variable frequency drives (VFD) / variable speed drives (VSD) Constant Pressure Pump Inverter can be user-programmed to “jump” those frequencies, thereby avoiding the increased vibration. Of course, this assumes that the process can accept not being driven at those speeds.