I just fixed my noisy pump yesterday. I found an idea on another forum and decided to try it.
If you have a 90* fitting on the exit side of the pump then change that to a straight fitting. Any RV parts shop will have one. The pump pulsates as it pumps and the 90* fitting makes the pulsing worse. Make sure you have a flexible line coming in and going out. Then isolate all the lines from each other or the RV it's self. My pump is under the sink in it's own compartment. I shortened some of the lines and the put some foam between them and each other as well as the floor and walls. I then zip tied some to a drain pipe. I also used some conduit clamps to hold one of the lines solid to the framing. I then made sure the pump was screwed to the floor tight on it's isolator pads.
After doing all that the pump is barely audible.
FWIW it's a Shurflo.
As a side note make sure you run every faucet and toilet till the banging subsides. Even the outside faucet if you have one.