how can i fix a loose fan ?

It's a Beko Oven and Grill 600 mm wide - you might have said.

The fan will very probably not be loose - the blades will be soft aluminium - one of them has become distorted with the heat and is touching something as it rotates - if the fan were loose on its shaft as you believe then it would not rotate.

Remove the cover of the fan blades - and examine what's going on - bend the blade(s) appropriately. A torch might help - or the oven light.

Good Luck...
July 2016