Monster Moto Shield - Fan humming

Hello Simhub,

First of all let me thank you for this software! I am already running a Dashboard, Rumble motors, Bass shakers and a Wind simulator. Building these is a hobby, I am not an expert just an economist. 

My question is in connection with Wind Sim. I am using arduino UNO and Monster Moto shield to avoid overheat as I have a 2x12v 10watt fans. It works fine however there is a humming noise (not sure if it is the FAN or the board) when it is running lower than ~90%. The forums propose to lower the frequency but I couldn't figure how to do this with Monster. I see the option in sketch for Adafruit and DK motorshield but the only option I see for monster is clockwise/counter clockwise. Am I in a dead end if I want to loose this noise? Shall I choose a less powerful fan and stay with adafruit or DK?

Thank you!