Try little pieces of soft self-stick insulating foam tape on the corners of the fans & holders, wherever mating occurs, also where the drive cage mates to the chassis.
Running the fans at 7v may help, just move the fan's black ground wire to mate with the psu's red 5v wire in the fan plug. Never move the wires on the psu side of the plug- It's easy to forget, start moving stuff around, and get a dead short- a bad, bad thing. If your silencers are like mine, they don't have pass-thru connectors, makes changing them around easier. For fans with pass-thru's, the output conector's voltages must be made to be the same as plugs on the psu, otherwise a short can occur if anything is plugged into one.
You might try something like B-Quiet or Dynamat to kill the vibrations, this stuff only makes a noticeable difference if your system is already quiet. I like to wrap it around the circumference of the fans, kills the vibration at the source, also big chunks on the side panels. YMMV. 2cooltek is marketing some Sonex acoustical panels, which I haven't tried, that may help. Putting a large acoustic ceiling tile on the wall behind the computer (or two walls if it's in a corner) helps kill reflected noise.
None of this stuff will make any huge difference, it gets progressively more difficult as the noise level diminishes. Unfortunately, it will never be silent, entirely too much air motion is required for cooling, and then there's the drives.... You're not keeping any old, noisy drives, are you?
And these guys sell shock mount adapters to put drives in 5-1/4" bays, rubber mounted, so they may help with noise. Haven't tried them.
Shock mounts