I have never used an FX5, but Eheim classic series filters, including the 2260, are probably the most silent filters on the market. However, the 2260 uses a big pump, and it sits on top of the filter (not submerged in a sump), so a slight humming noise is unavoidable. The 2260 I run on my 125G is close to our bedroom, and was too loud for my wife - a fan of classical music and an accomplished violinist with a very fine ear! I covered the interior of half of the cabinet with 3.5" fiber glass insulation material. You can buy a huge roll of that stuff at any hardware store for $10. I am also not using the top plastic cover for the filter, which serves no function and can ad a slight rattling noise. Now I can sit next to the tank, and basically not hear a thing!
A source of noise other than the pump are of course bubbles, and you need some of them for aeration. I've found that an Eheim diffuser is one of the most quiet outlet options for a canister filter while providing superb aeration for your tank. Using the insulation material and an Eheim diffuser (not sure if Fluval offers a similar device, but the Eheim diffuser can be used with any filter) would of course also quiet down an FX5.
Cabinet door removed to show insulation material.
Cabinet door and part of insulation material removed, eg for filter maintenance.