Raghu:
From my prior work with PWM controlled motor speed the PWM frequency needs to be high enough so that you can't hear it. I have also seen information that woild suggest that too low of frequency can cause vibration in the innards of the motor which could lead to pre-mature failure of connections inside the motor.

It also seems intuitive to me that if the motor is of a type with a commutator that the PWM frequency should be some factor at least 5 or 10 higher than the commutation rate at the highest RPM. I do not however have anything to back this up with.

I have routinely used 20 KHz as the PWM rate on the motors that I have built controllers for.

Hope this small amount of info helps.
Michael Karas