Viscous fan couplings can wear and become sloppy, so that the fan wobbles and rattles as it spins (perhaps taking a bite out of the radiator?)
I have "repaired" a couple by removing the nut on the shaft nose, fitting a suitable washer, and then refitting the nut.
This locks up the unit and makes it run true, but is now a fixed drive, not as makers intended, but it stops the rattle! Ideally, you should replace the unit.
Mr. Toad, July 2008
