Both of my sumps were/are silent. It all depends on how it is made and how it is installed that will make it silent or loud. My mag 9.5 is silent, and so is the sedra pump on my euro-reef skimmer. What you need to do to make your sump silent:
First, make sure that there are no sudden drop-offs like waterfalls. If you look here on my sump where water is dropped into first, there is a drop-off, but I angled a 2" piece of glass so that it gently lowers into the next compartment w/o a waterfall.
Second, make yourself a durso standpipe. go to http://www.dursostandpipes.com
and click on "popular modifications" and then on "hangon overflow". This is a super easy way to make a unit for $5 that will stop the toilet gurgle sound that traditional overflows make. I have two on my 60, and I can't hear a thing.
Lastly, make sure that your return lines from the main sump pump are positioned away from the water surface enough so that no water is splashed around. Some people like this sound, some people don't. I personally don't, and choose rather to point a PH
up so that it ripples the water. I have 4 returns on my tank, and all of them are silent.
When the power shuts off and the backflow syphon starts, it'll only drain about 1/2 inch before air is introduced to the system, thereby stoping the syphon. The water in my sump only rises by 1" ... so no danger of a possible sump overflow. All of these things will help to keep your sump silent. As a word of comparison... I used to have an Emperor 280 on my old 29 gal
setup... and that thing was LOUD compared to my sump now. 8)