[JRTJH]

Any screw or bolt that goes through the pump mounts to the floor of the trailer will, if it is too tight, transfer the vibrations (and noise) from the pump into the floor. That makes a very efficient "speaker" that amplifies the vibrations into annoying sound. I solved our "water pump speaker" by building a vibration plate isolator from plywood and soft "garage mat" cushion foam. Here is an old diagram of what I did. I've posted it several times over the past 6 or 7 years, but the original is still working for my purposes.