if you use a car pump for just that you want to run it on as low a voltage as you can get it to pump on , if not instead of a splutter of water it Will be a spray jet and the washer pumps are very noisy running flat out, you could try a wagon 24v one they have them exactly the same sizes and capaceties as car ones, or put a bulb in series of the 12v one that will slow it down and quieten it you could also put a coil around the motor and cool that as well before going out the back.
Peter
