Technical Why do these fans click and snap?

Any fan can eventually break, but at what point will it let go? Just be careful with used fans and avoid buying cheap crap. At the first sign of rust, something loose or if its real old, replace it. I have a Derale flex fan on my car and I'm happy with it.

 

A new plastic fan will be an old plastic fan very quickly. Plastics tend to leach plasticisers over time. I've had a plastic engine-driven fan come apart and knock an unsightly pimple in my BMW 2002's hood.

The understanding of metal fatigue is a relatively new thing. It is now completely possible to design a fatigue-proof steel fan, if you allow a bit for inevitable corrosion. Even a few years ago it was largely guesswork. It is also possible to check an old design of steel fan for its fatigue margin, and perhaps to reinforce it.