you know the shit you insulate your trunk with? you know to stop the rattling from subwoofer's deep bass?
you can wrap that around the fan.
for future reference, try buying fans with polymer castings the plastic will absorb most of the noise. after that the only noise would be the air whooshing thru the duct. that is easily silenced. you can also get a duct muffler/silencer if you like.
and before you do any of that, make sure you have rubber grommets, bungi cords to hang the fan or have it sit on something rubber or foamy. as the material will absorb the vibrations!!!!