Why does the sound change after I switch it on?

The sound of the machine changes and the suction is affected. I have checked the hose and head isn't blocked and they aren't. I've cleaned the filter and that hasn't made any difference. Any ideas?

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Does the unit have a belt that drives the brush? If so, have you checked to see if it were worn?

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Check the motor. It could be a dirty commutator or worn brushes. See https://therestartproject.org/wiki/Elect... in the Restart Wiki.

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I think it's clogged somewhere. It happened after I vacuumed a huge "dust bunny".

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