It was explained to me by Furuno support that there is a cooling fan in the pedestal that makes the noise you hear in standby. I have DRS25A-NXT but I assume the 12A-NXT are identical in this respect. I can't imagine that the voltage would make any difference.
What I ended up doing was installing a switch in the power supply to the radar so I could silence it when at the dock but using other electronics or in other situations where the noise is a bother. Because it is nearly instant on, you could leave it off if you aren't planning to use it. The only consideration is that if it was on when you booted up the MFDs, you will get a "radar missing" error when you turn it off so you need to cut the power to it first.