Do you have a permeate pump on your set-up? My set-up from air-water-ice has a little canister on top that uses the water supply pressure to help assist the post-filter water flow. It improves efficiency, but it's very loud. If you know for sure it's a solenoid, then never mind, but it's another option. Having said that, I don't know how to make it quiet other than by removing it! DonW's way of improving efficiency is by running the waste line of the RO to the input of a second RO membrane. After that there is very little waste produced.
Josh