Replacing the cap might help but it's likely the windings on the motor.
How old is the motor?
What I like to do is switch the speed from low to medium by changing the wires on the circuit board.
By doing this sometimes you can determine if it's the windings.
Start the motor after you change speed and listen to see if the sound changes.
In my experience a blower that has dirt build up in its vanes has less resistance on it and a lower work load.
