Probably a bit of tree debris/twig that fallen into the vent tract, this will knock on the fan blades as it rotates, I have had this happen in the past. It may be that the bearings are on the downward spiral but this is usually more of a rumble. Have a look under the bonnet for the intake to the interior fresh air system, feel around and see if theres anything in there !
Good Luck
SB, November 2006
