A sudden change of noise suggests a change in airflow - contrary to what some may think a vacuum motor runs faster and gives a higher pitched sound when the airway is blocked - it has no air to move and consequently much less load on the motor. When sucking properly the motor runs slower and makes a lower pitch sound.
A blocked airway will cause the motor to overheat and eventually cause the thermal trip to operate.
You do not say the model - Dysonmedic.com is quite useful for some machines - nice pictures.
Good luck...
Peccavi
December 2009
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix vacuum cleaners.