Aris, it is possible to silence a water pump completely. I am running an undervolted Eheim 1048 - it runs on 65V / 20Hz instead of on 230V / 50Hz. It's not even softmounted, and I can only hear it when it's completely quiet in the neighbourhood, and I'm putting my ear right next to the pump. The 20Hz hum is so low that it's barely audible.
However, it's not easy to accomplish. The trick requires some electronics for the undervolting part, and it doesn't work with all pumps. I'll post schematics and pictures in this forum as soon as I have some more time
I had an eheim also, cant remember if it was the 1048 or the step up from that (1280??), its been so long.
Anywho, i was able to get it to be very quiet, but there was always an audible "chug chug" low frequency noise that i was never totally able to eliminiate. I didnt have it completely suspended though(only soft mounted and encased in foam), mostly because i didnt want something dangling around inside. Too unstable for my likings.
Im sure if you put a lot of work into it, you could possible rig something up, but considering i could get the exact same performance with a lower system noise signiture for less money and less hassle with air.
For me it was never about the overclock, which is why i see most people using W/C setups, it was to try and achieve a quieter system, and in that aspect alone, air is superior.