I usually suggest a small bottle-brush to clean out the impeller well, as they are more efficient, and will not leave any bits of cotton behind. That being said, did you make sure the motor and impeller were cool before trying to start-up? If your AC300 is more than a year old, have you checked for uneven wear on the impeller shaft recently? Will the impeller start up if you reach in and give it an initial 'flick' with your fingertip?
G. A. Christian Bilou, Herpetologist
Founder/Director, Reptile Rescue Alberta
Past-President, Calgary Aquarium Society