My Janome 6600 - while I love it - is NOT quiet. To those that suggested foam or batting under the machine, this would not help the Janome. I had thought it was quiet but when I tried to get away with sewing while also on a conference call (boring board meeting) THEY heard the machine. In addition, the Janome is constantly beeping at you for something or other and the thread cutter is very noisy.
My Huskvarna Viking Lily 535 is quieter than the Janome ... how much quieter I don't remember. But it still beeps.
I agree with some of the others. If a foam mat under your current machines doesn't do the trick, try one of the older machines like a featherweight. If it's the beeping that is concerning you, perhaps a dealer/repair shop can turn that feature off.