Pla cooling fan pwm noise

Good day all
My 40mm pla cooling fan are connected to D9 on my ramps 1.4. Sometimes during a print while my fan change rpm, I get this pwm noise. Do anybody know what can be changed in Marlin and to what to make the stupid noise go away. I have an idea that it`s someting to do with: #define SOFT_PWM_SCALE 0 or #define FAST_PWM_FAN or #define FAN_SOFT_PWM, but am not sure. While the fan are full blast the sound goes away, but anything under around 200pwm, the sound comes back.

Hi,

Did you manage to work it out? I get the same behaviour with Yate Loon's D40SM-12C, they start to whine below 220 PWM

Unless you don't have the actual firmware, tailored for your printer at hand, it's a lot of work to adapt a "common" version of marlin to your printer, before you can play around with fan_PWM settings.
My best educated guess is : define fast_PWM_fan will cure the noise problem.
-Olaf