First check if there is air coming out the exhaust when running. There should be a significant amount of air coming out the back or into your ducting (presumably to the outside).
If not, you could have a broken or fallen off belt to the fan.
At the back there is a dark plastic cover. It has two screws holding it in. Remove these screws and the cover should lift off and unhook at the other end.
Check the belt. Mine was broken. This was the cause of exactly the same symptoms.
Good luck
Robert
July 2007