If it's the fan, try removing it from the PC, then peel the little sticker away from the back to expose the bearing, then add one or maybe two drops (no more!!) of 3in1 or light machine oil to the bearing and then replace the sticker. I've done this on a couple of occaions before now to cure rattly fans as a matter of course- good for the fans lifespan, as they tend to wear more quickly without adequate lubrication.
Just make really sure you don't put too much oil in or you'll end up with it all over the place
If you can feel lots of movement along the axis of the spindle then chances are the fan is borked, but if everything is nice and tight, then a bit of oil can be just the job.