Jo,

More than likely, you are correct that it's the fan...  However, you'll need to open the machine and see exactly where the noise is coming from, the cooling compartment or the lower compartment that houses the condensing unit.

If it makes noise all the time, chances are the fan in the cooling compartment is obstructed, the blade is bent or loose, etc. So check there...

If it only does it sometimes, I would look at the fan on the condensor plate and check to see it's free of obstructions, loose or bent, etc...