It sounds as though theres a section up on the motor. While you have the motor off, put your forefinger through the opening on the motor casing and gently rest the tip on the copper sections that the brushes run on. Turn the motor pulley with the other hand and you should feel whether a section is raised. If it is then although the motor may still run, it will spark alot and it will fail in the not too distant future.
Other than that, disturbing the brushes can cause them to click. If this is the case then the condition will rectify itself in a short while when the brushes settle down again.
steveD, April 2008