What I would do first is disconnect the power supply to the fan and open its enclosure. This is to make sure that there are no physical obstructions responsible for those growling sounds. This may not be the case but I've seen dead birds, bird nests, and all kinds of debris somehow stuck inside the fan assembly or fan blades slightly touching the enclosure. Also, check the shaft section between the fan and the motor, this area often gets contaminated as well.
If all looks fine and the noise persists after putting it back together, you'll most like need to replace the motor.
Let me know,