At low speeds, when the controller is at the slowest, yes, you can't hear them but turn the knob up to half and it's like the blower an a car, turn it up full and it's like a motor.
But I found that with the way I had placed my fan,
It was most efficient at the lowest speed, the moment I increased speeds the temps began to rise due to the turbulence inside the case.
And I used the fan only at full when I wanted to blow away all the accumulated dust inside the rig.