Good question. There are a couple of things to try. Have heard of some who have an attic or crawl space above the blowers. They piped the intake air to that space. Other thing to explore is noise deadening tiles. Can be very effective depending on blower location and structure.
How long have the blowers been installed? We've gotten complaints for lighting, beeping from the Dixmor timers horns, vacuums, but never from the blowers
I have seen large round disks placed over the intake side of the fans spaced out a few inches to allow air in but helps block the noise. You might also try a automotive undercoating on the outside of the fan housing.