If you're talking about the engine driven main fan and the respective bearing..... the viscous coupling will/can/should keep the fan itself from spinning too fast, BUT the pulley and bearing hub are still driven by the belt and will continue to turn at engine speed. With a mechanics stethascope (or long solid rod ie; extension ) you should be able to listen to all bearings and track down the noise, just beware of moving parts!!! Hope this helps and good luck!!
MV