Sometimes the way the water pump is mounted makes different noise levels when running... I have a section of RUG under mine...
My water pump has the Valve and hose to stick down into a winterize jug. Sometimes the value switch for this gets bumped and has a small air leak which makes a different noise in the pump...
This is also a good place to check if the freshwater tank is not picking up good - You can switch to the water jug and see how it works picking up water from it... If the motor sounds good doing this might be some sort of blockage towards the fresh water tank.

Roy Ken