If this noise is electrical hum from the motors, there's probably nothing you can do, as most ceiling fan motors have bearings, etc. that cant be lubricated. Make sure the fan is balanced. Replace the fan.
If the noise is the blades "whooshing" in the air: 1) clean the blades 2) balance the blades 3) if the fans are the ceiling hugger type, add extensions to move then farther from the ceiling 4) run them on "Low"