I don't know anything about this particular brand/model, but I'd imagine that's just normal arcing between the carbon-brushes and the armature - the carbons and/or armature may need replacing if the sparking is severe.
I've known a number of people who've said 'Oh you collect vacuum cleaners? You could have had ours - we threw it away the other week because we could see sparks in the motor...'
I myself stopped using many cleaners when I was younger because I was alarmed by the 'green flashes' you could sometimes see, and feared I was about to be electrocuted!