I had this problem and it's one of two things which is causing the problem. Either the spindle which the fan is rotating on is allowing the fan to slide when the grill gets hot OR the metal frame which holds the fan is expanding with the heat. Either of the above situations forces the an blades to come into contact with the cover (for the fan) which is causing the rattle.
Simply take the oven out which involves removing the four screws either side of the open front door and sliding the oven out.
When the pver is out you will see the offending fan from the top of the oven. Remove the 10 screws which secure the cover to the fan. Give the fan a good clean and try to create a ridge on the fan spindle by using a junior hack saw or similar. When the ridge has been put in place then put the oven all back together again and use it...the rattle should have gone.
Peter Duncan
April 2008