Vacuum motors are cooled with the air drawn through them - they can get very hot and risk catching fire if they're not cooled - so they have an overheat cut-out switch to shut the motor down if it gets too hot. The switch will self re-set after an interval.
The load on a vacuum cleaner is the air it shifts - if the hose is blocked or you cover the air inlet with your hand then no air can get in - the motor has no load and it speeds up - you can hear a higher pitched note - but it also has no cooling air so it's just a matter of time before the safety switch operates.
When the airflow is restored then the motor has proper work to do and you can hear the pitch change to a lower note.
Strangely a vacuum motor will use slightly less power hen the hose is blocked.
Your issue is caused by the white ball blocking the air flow - either that or an overly sensitive thermal cut-out switch.
There will also be an internal filter to stop any dust and dirt entering the motor - you might check it and give it a clean if it's dirty.
Good luck...
Peccavi
July 2013