I'd suspect that model has a hydraulic cooling fan, the belt driven ones can get a rattle from the belt or bearings in the cooling fan gear box.It could be a lot of different things. You are going to need to supply a bit more detail:
Does it vary with load?
Where does it seem to be coming from?
When did it start doing it?
How many hours are on the machine?
Are there any other symptoms?
When was it serviced last?
-SR