Does anyone know a silent UPS?

Looking for a UPS that is almost silent? Any ideas? It's just for a NAS...

I use this one. It's dead silent.

thanks, so basically you use it to shut down your PC when power out, because the runtime is <10 mins.

Yep. Unless you plan on spending a lot of money on a UPS, that's going to be about the average up-time after power loss.