In Low Rpm i listen the Coil of fans - Why?


Hello to all!
I have 5 Scythe SF21 G. They are 12V x 0.24A = 2.88W each.
I use AEROCOOL F6XT.
I have connect one fan per channel.

When i set the speed at Low Or Medium i can hear the Coil of the fans.
Without AEROCOOL F6XT, or with the fans at max rpm the coils stop the noise.

Do you know something about that?

Thanks.