Thermo viscous fans are a bit like torque converters or to be more exact the poor relation of them, the fluid coupling.
When first started the fan runs at engine speed until the fluid is forced outwards within the centre hub, then the fan will runs at a max of about 1500rpm. When the little bimetallic spring on the front gets too hot a valve opens and allows the fluid back in to the centreo f the hub, where it transmits drive to the fan. The fan may then run at about 2,000-2,5000 rpm but no more.
So if you hear the fan once the engine has started that's ok, but the noise should subside, you only hear it a little then when the enhine gets too hot.
Viscoous fans are meant to absorb only half the power of a fixed blade fan.