I had the same problem and phoned Dyson as it was still under warranty. They sent me a replacement filter and it fixed the problem. Never had a squeak since.
Gail, February 2008
Hi, go to www.sparesaver.com and look at their 'Help & Support' section. They have superb full instructions including colour photographs of troubleshooting Dyson problems.
Jack, January 2008
Have you tried cleaning the spindle and replacing the belt?