It would cost about $10-$15 in plumbing supplies, but if you were to just stick a zig-zag or two in the long stright sections of the drain pipe, it will REALLY drop the noise in a big way from the decrease in velocity of the falling fluid.
Also, if anybody wants to make a totally silent overflow, make a large expansion area near the bottom, like 1.5" or 2" drain tube into a bell reducer up to 4+" tubeing for a 8"-14" section of tubeing. At the bottom of this 4" pipe, use another bell reducer to drop the size back to the original tubeing size, or even slightly smaller. Make 2 90deg bends in the tubeing so it curves back up parallel with the drain tube. End this tube about 3/4 of the way up the section of large chamber.
Kinda bulky, but totally silent, at least on the lower end. To make the upper end silent, a simple U-bend as soon as possible from the point it drains will do the trick.
If you need diagrams or something, let me know and I can sketch them in paint.
