There are a couple of mods I know of to help quiet pumps.
I did these to my old trailer, and it cut the racket/vibration in half:
I removed the mounting bolts, and remounted the pump to the floor with clear plastic tubing under the bolts...to act as a cushion. It stopped a lot of vibration. Cushioning the pump/floor contact area with any soft/flexible material would probably help.
I also added some padding between the plastic water lines in the area to stop them from chattering.
I then used styrofoam insulation to sound proof the area surrounding the pump.
Another mod that people swear by is lengthening the water line into a longer coil. Not sure on the theory of it, however.
Other folks add a surge tank to keep the pump from kicking on so often, and it adds a little extra water capacity to your trailer.
You can also buy much quieter aftermarket pumps.
Pete (Jolly Mon)
2014 Jayco Whitehawk 24 RBS
2004 Nissan Titan LE, 4x4