I have found that it isn't always the pump that is noisy, but is usually associated with the water hammer effect on the lines on the outlet side of the pump. It may help to try repositioning the lines or moving them around while the pump is running to see if the noise goes away or is minimized. Once you find the area of line that is hammering/vibrating either place a piece of foam pipe insulation around the line to isolate it, or use a nylon zip tie to anchor the line to something firm to change its position slightly. All it takes is for a section of a line to rub on something and cause annoying sounds. In some cases, it could be a section of line far away from the pump.
This approach has worked for me in more than one trailer.