I converted a Jabsco to use tank water several years ago. It is now barely audible.
I used a solenoid and a backflow preventer identical to the set up my Sea Era came with when I re-did the aft head.
To make the Jabsco flush quietly, I removed the intake impeller and pipe plugged off the ports.
The water solenoid makes as much noise as the flushing motor. (Unless there is sizeable waste to chop up)
It's an easy conversion.