I experienced this sound after getting my Honda City's AC & blower serviced (with 60K kms maintenance). It really really is annoying - At the slowest level, you get to hear a continuous noise from the motor - (sounds like the motor's rotor rubbing with the brush).
The noise becomes less audible when you speed up the blower - may be due to the sound of air blowing out, and naturally the motor speeds up so does its inherent sound. Interestingly, the noise doesn't amplify with increasing the speed of the fan, Or may be the sound of motor itself and the air blowing is more than it.
But it is really really bad! Honda would say - ye to jee nayee gariyoun main se bhi aati hai!! and you feel like keeeling em!