If the airway is blocked the motor will "race" as in run much faster with a higher pitched sound.
If you can make this change of pitch yourself by blocking off the airway with your hand then the airway is probably clear.
That leaves the motor itself - either shot bearings or brushes.
Dysonmedic.com has a comprehensive article about replacing the motor on a DC07 - about 700 pictures!
Good luck...
Peccavi, January 2013
