There are two basic ways of reducing noise generated by centrifugal pumps, these are lagging or using an acoustic enclosure.
Lagging consists of fitting the pump with an acoustic insulating blanket that absorbs the some of the generated noise. The blanked must be carefully fitted to ensure optimum noise reduction. This applies as much to first fit in the factory as it does to re-fitting after pump overhaul. Lagging cannot eliminate noise but can significantly reduce it and is a relatively low cost option that does not add bulk or complexity to the pump assembly.
Acoustic enclosures are structures built around the pump (or pump and gearbox in some instances) that isolate the equipment from its surroundings. Their walls are lined with acoustic absorbent materials and they can offer a significant reduction in noise output. However they are expensive, isolate the equipment from view and, depending on the environment, can create a hazardous area requiring higher protection rated instrumentation and other electrical devices. If the pump is operating at temperature, then forced ventilation may also be required to control the environment inside the enclosure.