Donna - as Danny says it could be that something is stuck on the brushbar. Have a look for any obstructions around there.
If not I expect it's the motor. The brushes only have a finite life and when they wear out it will make a noise. Dyson can do a full service on your machine for £69 (including replacing the motor if necessary).
If neither of those things are the problem try asking on the unofficial Dyson forum: www.dysonforum.com
Roger
November 2009